Charging for parking makes sense if you have a dedicated parking facility in an area where parking is a limited resource, multiple audiences are competing for space, and mass transit options exist. Take downtown Nashville, for example, with the HOF, Predators hockey, the Ryman, and 8,342,357 honky tonks.
When you have what's basically an unlimited resource, everyone who wants to use it is attending only your event, and you can reasonably assume driving is the only way they'll get there, don't charge for the parking. You slow everyone down, you have to pay people to accept the cash payment, you have to accept that they may be pocketing part of your fee and that some of the money may be counterfeit*, and it just doesn't sit well with the fans.
If you're absolutely dependent on that income, just raise the up front ticket price. You may fool people the first time into thinking you're offering a reasonably priced ticket but they'll remember the hassle and mark-up for parking, and it won't be fondly.
* Take two $20 and a $1 bill. Tear the about an inch off each end of the $1; leave the serial numbers intact. Tear the left end off one $20, the right end off the other. Tape those $20 ends on the $1. The two $20 are still legal tender. Pay for your parking with the $1. This works really well in a setting where an inexperienced kid is trying to conduct a high volume of cash transactions as quickly as possible, so he's looking only at the numbers.
