Always best to check a CG's website to check on those "hidden charges". We've seen $2 charge per bicycle, $3 to $5 charge for 50A service as well as a charge for sewer. Some 'resorts' will charge a fee(as high as $7 per person, per day)for visitors to your campsite and a fee for their vehicle.
Some charge for any kind of 'premium' site as well. Broad spectrum on additional fees they charge for...so check ahead of time so there's no surprises.
If you plan on traveling a lot, move from CG to CG, then Passport America is a good money saver. Sometimes the CG will honor the 1/2 off for additional days if asked. If your more of a destination camper, then Good Sam pays a 10% dividend...otherwise, we've found their measly 10% is lost in additional fuel costs to get to the CG.
By far, the best bargain we've found is the Senior Pass...so far, it's been 38 National Parks for the grand total of $10 bucks!
Although we've only stayed overnight twice, the most expensive 'free' stays have both been at Wal-Mart.