Reducing material costs to the organizers
Make the stakes high enough, and the organizers won't have to pay one thin dime towards the mechs -- the competitors themselves will pay to field them.
This is how sports such as NASCAR already work: There's a shiny pot awaiting the winner, but to get there you, not NASCAR, have to buy a car and put a driver in the seat.
As you've described it, this is a hugely-attended sport. The organizers are necessarily making an absolute killing from ticket sales, advertising, and broadcast rights. They could easily put some of that toward a ridiculously huge cash pot awaiting the victor, as well as the various other rewards that would necessarily come with being a winner (spokesmech offers, appearances at events, etc.). Get some corporate sponsorship by slapping a big sticker on the mech's side (a la NASCAR cars), and you've got even more of the cost of fielding these things covered.
Reducing the risk to life
Remote-control is the simplest solution here, short of turning to simulated battles, and actually is not hard to accomplish while defeating jamming tech -- within the confines of a sport, that is.
First off, shield the arena itself from EM. This would block any external transmitters from being able to broadcast any signal to any mech. Then, the organizers themselves provide each mech/team with access to a dedicated transmitter in the arena and a randomly-assigned frequency; on fight day, the mech's operators/mechanics would program that frequency into the mech, and then plug the remote controls into the cables that feed said transmitter. Add a few sensors in the arena to detect "rogue" signals from any mech that aren't on its assigned frequency (with immediate power-down of all mechs and forfeiture of the match the consequences of breach of this rule), and you've got a very well-hardened system to prevent jamming. (Spoofing is trivial to defeat: Encrypt the signal between controller and mech. Any signal that doesn't decrypt properly gets discarded (and, of course, logged in case it is evidence of attempted tampering).)
For a little extra assurance, the organizers can mandate a "status" signal that the mechs broadcast back toward their transmitters (again on their assigned frequency); any failure of this signal (e.g. via damage to the mech's radio gear, or to the transmitter itself) would be immediately detected by the organizers, and all transmitters promptly shut down while the problem is investigated. A randomly-generated encryption key, provided to the mech's owners on fight day, would prevent spoofing this signal as well as revealing the cheaters trying to do so (and thus causing them to forfeit the match). Do the same thing from the controllers themselves, and now the organizers can even detect tampering of or damage to the cables connecting remotes to transmitters.
Add in surprise inspections of equipment to look for any sort of jamming technology, and the risks and costs of getting caught make simply fielding a better mech the most cost-effective way of winning matches and, ultimately, claiming the title of Top Mech.
Bonus: Alternative to reduce the risk to life
Similar to NASCAR regulations, mandate that pilots be enclosed within ultra-hardened pods. Any damage to/breach of these pods would be considered the "death" of the relevant mech; the status of these pods would be monitored remotely by the organizers, and an immediate "stop" command sent to all mechs on the field once one pod is damaged/breached. (In the case of a team match, the stop might be temporary: The "killed" mech might be removed from the field to save the pilot, and then the match resumed with the remaining competitors.)
Additionally, give pilots a "panic button" they can hit if they ever feel they are in danger and unable to safely proceed; this would be analogous to a NASCAR driver leaving the race by pulling off the track, or the crew chief declaring that a car stopping at a pit-stop is unable to continue the race. While they've forfeited this match, they've lived to fight another day in another match. (And as a bonus, the button could be hit in a losing match to preserve the mech itself, thus reducing property damage as well.)
There's no way these pods could be designed to be fool-proof, and thus there's still the risk of a pilot being injured or even killed -- just like a NASCAR driver can be injured or killed on the race track. Keeping the pilots inside the mechs necessarily means that they will face this risk; the best you can do is to reduce it as much as possible while still keeping as much of the original "feel" of the sport as you can -- no matter how good the pyrotechnics, there will invariably be people who simply tune the whole sport out entirely if the fights are reduced to simulations in any way. A blood-thirsty spectator demands a blood-thirsty sport, not pretty-but-mostly-harmless explosions.
