All of their rentals are for unlimited miles. Last month I had a successful bid on a minivan pick up in Ft. Lauderdale. I got a great Dodge Grand Caravan at Alamo. Price was $20 per day plus their processing fee of under $6 per day. The total for my 8 day rental was under $200 with unlimited miles.
A word of advice, Priceline will list an amount that they say has only a very small percentage of being accepted, in an attempt to get you to increase your bid. Ignore that. Bid lower. Both times I got a priceline car I ignored the warning, bid lower than what it said was the lowest acceptable bid, and the bid was accepted.