I think the reason why paintball is seen as expensive is because of startup costs and any non walkon/tournament events.
To get an entire setup, you're looking at spending at least £300 on gear, which is a pretty large sum of money! However, people don't see that this will most likely carry you through the next few years of playing, unless of course you decide to replace some items. Rental days also make people think that it's expensive. Your regular rental day will see you paying £8 per 100 balls ( or somewhere in that region ) and a case of paint could set you back three figures depending upon where you are. If you're at a walk on like Diamond wars or you head down to CPPS to play, you can pick up a case for £25, albeit one case might not do you, it's cheaper in comparison to rental days. Whilst it's seen as expensive, it really isn't once you get past the rental stage of paintball.