Magic is expensive
I have a setting where gold is valuable because it must be consumed to create any magical effect. If creating a ton of iron magically costs significantly more gold than buying and transporting that ton of iron, then no one need to worry about the economic impact: no one could make money off that, so it will never be done at scale. Magic is useful for its flexibility, but mundane methods are always more cost effective.
Magic is constrained by economics
In economic theory, prices are determined by "general equilibrium" which depends on the quantities supplied and demanded for any commodity, as well as the exchange ratios to other commodities.
- In a world where magic is constrained by principles like "equivalent exchange" in an economic sense (what in economics we'd call a "no arbitrage constraint"), there's no direct way to make money off of magic (though there might still be a somewhat lucrative business turning 500 dollars of iron into 499 dollars of gold, for example).
- If magic has myriad of hard to detect indirect effects, it could sterilize the economic impact of each casting. The simplest example is "creating" a pile of gold coins really just skims a bit of gold coins off every other pile of gold coins in the world. Total supply of gold remains the same, so prices will stay relatively stable. However since it adversely impacts the rich, they will collectively spend a lot of money hunting down anyone who might cast a spell like that.
Magic is above the mortal economy
If magic users can create a million tons of gold per day, and trading only with other magic users consuming a million tons of gold per day, it doesn't really impact the mundane users' economy. Nothing a mundane user does will ever be relevant to the magic users who are effectively gods.