Priceline will tell you the lowest bid they will accept for every class of car if you have a bid accepted for a hotel in most cities. I didn't get this point until I called and they explained. I was trying to bid lower than their suggestion. If you get a hotel and want different dates for the rental car, then simply change them. You will also see a price for every type. The prices will change day to day and can get very low toward the last possible day. But always compare with Hotwire. I booked for Phoenix. The price on Hotwire is actually lower for the car ($13 vs $14 per day for compact or mid-size) and Priceline's booking fee is twice that of Hotwire (so on the seven day rental, Priceline was higher by $20 overall). BTW the Hotwire car is from Avis. My experience with Priceline is usually Alamo and sometimes budget.
The car rental price that Priceline shows on a winning hotel bid is not the lowest they will accept, in fact there's no guarantee it will be accepted at all. I've bid lower than they price they showed and gotten a car and bid what they showed and not gotten a car.
I've received cars from Budget, Alamo, Hertz, and Avis through Priceline, I only used Hotwire once and received Hertz.