Mark C
As has been pointed out above, and I'm sure you understand, sites have costs to cover, staff, rent, equipment, catering, insurance, rates, etc. etc. hence why they charge so much for paint.
Walkon days are an excellent way to get going if you are new to the game.
The single most important piece of kit is a decent set of goggles, your eyesight can not be replaced. After that, for walkons, you can use just about anything, from an old skool pump to the latest tournament marker. The choice is yours.
A second hand Tipmann or similar is a cheap way to get started, a 3K air system (or CO2 if your site uses this) some sites will lend you air/CO2 systems if you ask. Battle pack and a few pots. If you want to buy new, I can recomend LiPS (see their forum in the sponsors section) with service second to none.
You don't need all the shiney gear just to have fun, you can wear old clothes, or get some combats from a surplus store etc.
It would be worth your while ringing round the sites local to you to find out what they have on offer, you may even be able to pick up a job marshalling or some site work to help pay for it all.
Hope that helps.