Depends on the place. Most do by the hour with say two horses allowed, then an extra amount for every extra horse. Also some charge for use of jumps, some don't.
our local school is £15 for an hour for two horses then £3 per extra horse, max 4 aloud at a time, includes the use of BSJA show jumps, but not lights which are on a meter, and you can use half or all the lights at £1 or £2 pounds