How would you monitor the expenditure?
Require all participants to submit contractual details to some monitoring agency.
Expel anyone found paying under the table, would probably happen to a couple of teams after a handful of years.
Other sports have had budget cap scandals.
This suggests that it is possible to police and that cheats will be caught from time to time.
People will ALWAYS cheat.
F1 already has technical regulations. They manage to monitor then some of the time.