3000 BB's was enough for me last year, and that was playing all day Sat, Sat night game, & Sun until end ex @16:00-ish - about £30-35 for food can do it, but yeah I had forgotten about transport. You can't use pyro's that burn (fire hazard) so no expense there. If you look back in this thread, Ed and I were discussing how much it could be done for on a tight budget and he said ignore transport because he had that sorted.
If you have a really heavy trigger finger and manage to stay in contact with enemy without getting hit for a lot longer than average, then just maybe you could get through £30-40's worth of BB's, but the site is huge. I mean really huge, like big enough to get lost in, so you spend a fair bit of time patrolling to where you want to get to (because, if you just leg it along the fastest routes you can find on the map, you're going to walk into an ambush). Unless, of course, you're shooting hand polished Faberge BB's?