Right, upping the price reduces the number of people likely to attend. However you can do things that you can include which will work out improving your profit margin.
EG: The sandpit was £30 last time I played for the day, that's £10 more than most of the other sites I played, they included lunch which their competitor didn't so
Sandpit 5 to 6 hours play, plus a hotdog, baked beans, a handful of nachos and some catering cheese (probably cost less than 2.50 pp as you'd be getting catering size from a cash and carry). £30 That's an extra £7.50 per person. although they spank a ton of money on pyro and fuel for their motor pool and loads of staff. Although Andy the owner is essentially a full time employee.
Competitor 5 to 6 hours play, no lunch. £20, lunch is extra, £2 for a burger, £1.50 chips, £2 cheesy chips. £1 cans of drink.
Don't forget, if you only open one weekend day every other week that's 26 possible game days. For a good day's play at a CQB place like Red 1 or the now closed SWWG I would probably be happy paying £20 for a day's play. But would be disappointed if it was rammed to the point that every game was a stalemate.
If you do a weekday evening every week that's about 50 half days. Taking out a few for Christmas/new year although you may find people are interested. Half the days play cost would be fair, £10.
SWWG did juniors only nights and weekend days on the alternate weekends to the open days. The juniors days included lunch and essentially they supervised the kids, I think they even did a bit of training. So that increased turnover.
Back to the original post, its way to hard to figure out a perfect number of people or perfect size, there are so many factors that are in play.