What Arthur said, hardly anyone really sticks to the budget, and half the time the people who do stick to it are running the nicest cars....
I've done a few shitbox rallies, and I've always had shitboxes yet we've spent well over a grand per car, even if the car was free

(well, I think we were in budget once....)
And then there were people who managed to snap an 8yo luxo cruiser for 950, ready to go

There were years where they had a stripper pole, and if your car was deemed to be over budget, you'd have to dance on the pole for a little bit. One 70yo guy did it in his undies...
As a different rally I've done said in their rules, "we encourage you to not spend more than £250, but we can't go round scruteneering everyone, so as long as your car is on the spirit of the event, we don't care. And even if it isn't, your punishment will be in the form of mockery by everyone else on the event for the entire duration of the event"