We went in a group of 5 last year and all used a similar amount. 1000 on the Fridays mini games, 4000 On Saturdays UK verses, and then 4000 on Sundays big game. We were uncapped and trigger happy.
Most of the vendors will have paint in several different grades, and reasonably cheap.
I think the hardest choice to make is hard cheap paint or tournament paint that breaks easy. We always go for the tournament paint harder for the other side to cheat because that **** is hard to wipe off.
Last year a 2000 box of Empire Evil was £28, but the more you buy the cheaper you get it.
And its site only paint in case you did not know! Every car gets searched and they will ask you to open your bags.