Anyway, contrary to rogery post, most hotels at Rosemont still charge for parking, except it may be $10 a day instead of $20 a day in town. Except Hyatt Rosement (not to confuse with Hyatt Regency Rosemont which is the big convention hotel very close to the rail station) has free parking, all other hotels charge you. There is a trick though, if you dont use your car during your stay, i.e. get in and out garage ONLY ONCE during your whole stay, you can park at Rosemont Convention Center garage - it is a cash payment upfront, $10 last time I used it. Tell the entrance attendent you stay at the hotel next to it - the attendant will direct you park all the way toward the entrance of the hotel. You then leave your car there for as long as you need to. Just take the walk bridge to the hotel to get out the garage. There is no check out at the exit so you just drive out when you are ready to leave.
Park and Ride fill up very early in the morning. Unless you intend to get up as early as commuters, otherwise, forget about finding a place there. Also you cannot park overnight at Park and Ride - if you do so, you will get a ticket, trust me.
You can get hotels very reasonably via bidding on Priceline. Far cheaper than you go to a Motel 6 or whatever motel, and much much nicer.
For more info on current winning bids, you may want to go to http//www.biddingfortravel.com to check out Chicago thread.
That site is a good reference for priceline bidding.