What a bargain you got, eh?
Do they have many empty sites from people who pay to rent a site and then just use the dump?
People used to do something similar at Hillsborough SP in Florida when the pool was open. It was $6 per vehicle to get into the park plus $4 per person to use the pool.
But for $24/night you could rent a campsite for up to 8 people and you'd not be charged the vehicle entry fee and could use the pool for free.
If you had 8 people going swimming it would have been $38 (8 x $4 pp + $6 entry fee = $38) . It was less expensive to get a camp site, so you'd see some empty spots when the pool was open.
I'm not sure if it's still that way at HSP.