ATC is a club. You pay a fixed fee per month, cost depending on which plane you want to fly (the 172SP costs a little more per month than the 172N/P). There is a contract involved, basically stating that there is a 'soft' limit of 50 hours over 6 months. Fuel is not included in the price, but there is a 'fuel club' run by the airfield. $18 per month gives you a $1/gal discount on fuel. So, basically, a half-tank fill-up in the 'SP pays for the monthly charge.
Potomac is in the FRZ, so anyone wanting to fly out of there needs to go through a relatively painless (no, really!) clearance procedure. Once you're cleared, as long as you play by the rules (and they're not really as tough as they're made out to be), they pretty much let you do what you want. The owner of Potomac, Dave, is a great guy to deal with. He's got a C337 based right there, and he is very active in the anti-ADIZ/FRZ lobby.