I'm in the middle of creating my own role-play game set in a fantasy/sci-fi setting where the society is attacked fairly regular by monsters and the like; Due to this I'm having trouble with the pricing of weapons etcetera while keeping it balanced and affordable for players so they have a good variety of weapons to try out and buy.
So, slightly unhelpful answer: they'll cost enough to keep your game balanced.
More practical answer: economies of scale will make things cheaper and easier to come by, though do be aware of tropes like Adam Smith Hates Your Guts or No Hero Discount. As far as killing monsters goes everyone is going to be happier when there are more people available and equipped for dealing with incursions.
Note though that even in areas with quite relaxed weapon ownership laws, you can't buy just anything you want, and all the nicest stuff is kept for the local military and possibly law enforcement.
Also note that in areas where citizens may be part of a militia as conscripts or volunteers they may be provided with suitable weapons by the government because again, it is in everyone's interests for the militia to be armed appropriately even if they're quite poor. Arms, or sometimes ammunition provided in this way may only be accessible in times of emergency, however. Real world people generally can't just go full murderhobo on a whim, and it has been observed that militia are more likely to shoot themselves or their friends, family and neighbours if they have ready access to guns and ammunition whenever they fancy it. Have a read up on how Swiss and Israeli militaries operate for more on this subject.
An advanced and stable society which needs to defend itself is more likely to want a well organised, trained and equipped army or militia than a bunch of mercenary murderhoboes accountable to no-one.
The price of the weaponry would depend on the severity of the most common, or most prevalent, threat.
What I mean by this is that if you can take out most threats with a relatively low-powered firearm, the need for more powerful firearms is reduced. There is no need to use a 50.cal sniper to kill a common goblin you could kill just as easily with a 9mm pistol, for example. However, you might need that 50.cal sniper if you regularly go up against dragons to pierce their armoured hide.
Supply and demand would tell us that, as there is less market for these more powerful weapons, there would be less of them produced and their price would be higher (as there is still some small market for them and due to the skill involved in their production). On the contrary, less powerful (thusly easier to produce and lower in cost) firearms that are “good enough” to deal with the most common threats would be in high supply and high demand, making them cheaper.
However, if your most common threat was dragons, 9mm pistols become almost worthless as no one really wants one and there aren’t many being produced anyway. The 50.cal rifles would then be in high demand and high supply (as manufacturers prioritise on making these rather than anything less powerful), thusly their price would drop significantly.
Just a quick note, in terms of balancing the price for your players, consider that the price would change if the most common, or the most prevalent, threat changed. So at low levels, everything may be running as normal. However, as the campaign progresses and the players start levelling up and facing more dangerous threats, the average person would become more worried about these more dangerous threats and demand higher power weapons, thusly reducing the cost and making them more affordable to players at higher levels.
Further, consider that militaries and law enforcement would likely prevent most people from owning military-grade weaponry if there was no reason for them to. If everyone owned high-end fully-automatic rifles, it may be impossible to prevent a riot or civil war through force. However, as the need arose for more powerful weapons to deal with these highly dangerous threats, military and law enforcement might become far more lenient on who can own these weapons.
If the technological level allows, they're probably slightly cheaper, but much easier available. Most first world countries have hurdles to stop wide-spread gun control - from blacklisting like the USA to whitelisting like most of Europe.
In your world, wide-spread gun ownership will be required because everyone need to be able to defend themselves. Therefore, they'll be widely available. If your monsters have some specific weaknesses, then the standard ammo will be specialized for them. For example, if there's a hard shell on them, then they're probably Armor Piercing to a sufficient degree.
Benefits of mass production in an post-industrial revolution society will make them a bit cheaper than they are in our world.
This is of course dependent on wider society. If there's sufficient wizards around (like 1 in 10 or so), then perhaps they just have a duty to protect people, obsoleting general gun ownership because it's very possible to just always have a wizard around.
First, the obvious caveat to what I'm about to say is that the price needs to be whatever it has to be to keep the game balanced.
But from a worldbuilding perspective, the price of anti-monster guns depend on just how frequently monsters attack. I say "anti-monster guns" because guns in general are good for more than just shooting monsters, and so their price is harder to predict.
The price of anti-monster guns will be an inverse bell-curve. It'll start out high, as monster attacks are rare; few anti-monster guns will be made if monster attacks are rare. As monster attacks become more common, demand for anti-monster guns will increase, and so will supply, and price will fall. Eventually though, attacks become so common that society can't survive. If the attacks are too common, supply chains break down, and guns have to be made one-at-a-time in small shops again, which drives the price up.
From a simple economic perspective, the laws of supply and demand would work to decrease the cost of weaponry. Since there will be a great demand for monster killing weaponry, suppliers will boost output to service the demand, other suppliers will enter the market and a host of other goods and services related to monster killing will also appear (everything from holsters to training and bodyguards for people incapable or unwilling of using the weaponry).
This actually applies regardless of what sorts of weapons are being discussed, so if killing monsters requires silver coated knives or anti tank missiles, the general trend will be for an increase in supply and a downward pressure on prices.
There are, of course, many factors which would serve to disrupt the supply/demand equation. The monster hunter's guild could pressure the government (or suborn officials) to prevent the sale of weapons to civilians, driving the price upwards (as demand is still high, but supply has artificially been choked off). The development of some sort of superior alternative technology that makes current generations of monster killing weapons obsolete will have the opposite effect. Similarly, the number of monsters available will also affect the demand for weapons, and thus the price. A massive kill off of monsters will make monster killing weapons a glut on the market, collapsing the prices.
So in general, the appearance of large numbers of monsters will create a demand for monster killing weapons, the price will depend to a large extent on how the supply of these weapons is dealt with.