If you want a good night's sleep then you're definitely going to need something under you. (do you know what the likely night temps are and what your sleeping bag is rated for?)
REI rents gear, too.
If this is the one and only night, or two, you expect to be camping in the next decade then I wouldn't spend much on a pad. I'd first ask friends and family if anyone has any kind of a Thermarest pad, Aerobed, egg crate foam or even the cheapest air mattress -- that you can borrow.
Alternatively: Wal-Mart or some other cheap box or surplus store for a super cheap air mattress (you have duct tape in the car?).
If it were me and we're talking about just one weekend, then I'd take an extra sleeping bag to put under or a bed spread - something bulky.
More important in a group camp:
E A R P L U G S. Here are several options at REI. You could take a long thin foam pad and fold it over to cushion the most sensitive parts (for instance, my feet don't need a pad):
http://www.rei.com/outlet/search?cat=8001&cat=8001&cat=22000001&cat=22000021&cat=29340995&hist=cat%2C22000001%3ACamping+%26+Hiking%5Ecat%2C22000021%3ASleeping+Bags%5Ecat%2C29340995%3ASleeping+Pads
If you want to invest (and get a great deal) on a super comfortable and durable car camping mattress then LL Bean as the Thermarest DreamTime on sale (and free shipping on purchases over $75. I can attest to the comfort of the DreamTime, it's what I sleep on in my teardrop trailer):
http://www.llbean.com/llb/shop/67070?parentCategory=505122&feat=505122-tn&cat4=500305Good luck!