If you aren't afraid to do a bit of modding, you can easily change the amount to bet. Even without the module system. Just go into menus.txt and look for menu_tournament_bet and change the appropriate numbers.
The specific bit you're looking for is:
mno_bet_100_denars 2 2149 2 1224979098644774912 360287970189639680 30 2 1224979098644774912 100 100_denars. 2 2133 2 144115188075855905 100 2060 1 864691128455135340 . mno_bet_50_denars 2 2149 2 1224979098644774912 360287970189639680 30 2 1224979098644774912 50 50_denars. 2 2133 2 144115188075855905 50 2060 1 864691128455135340 . mno_bet_20_denars 2 2149 2 1224979098644774912 360287970189639680 30 2 1224979098644774912 20 20_denars. 2 2133 2 144115188075855905 20 2060 1 864691128455135340 . mno_bet_10_denars 2 2149 2 1224979098644774912 360287970189639680 30 2 1224979098644774912 10 10_denars. 2 2133 2 144115188075855905 10 2060 1 864691128455135340 . mno_bet_5_denars 2 2149 2 1224979098644774912 360287970189639680 30 2 1224979098644774912 5 5_denars. 2 2133 2 144115188075855905 5 2060 1 864691128455135340
Numbers to change highlighted in
red. These are the 5 different arena bets - just change them to whatever you like. First number is the name of that menu option. Second number is the amount that you're actually betting (saved to $temp for next steps). Third is the text that displays, so you know how much you're betting. Fourth is the amount required to be able to see the option. I don't think you really need to change the first one, since it's just the name and isn't displayed, but I did anyway for consistency.
I multiplied all mine by 10, so the lowest is 50 and highest is 1000. Given the new costs of armor, it seems much more reasonable. Did anyone EVER bet 5, anyway? I mean honestly... 5 denars? What kind of pauper needs to bet that?
This works perfectly - all the other instances dealing with tournament bets reference the numbers set here - the one that takes, gives, all the displaying ones, etc.
-edit- Just figured I should say this is for 1.003, since some of my old posts have a habit of staying around between versions.

It works for .960 too.
Link to other tweaks.