I spent four days in Missoula in 2012 and found the Budget Inn just off I-90 (can't recall the exit but I think I recorded it in my trip report) excellent. From what I recall, with the coupon, it was less than $50 per night, all inclusive. I did not book, I just turned up.
Lifey
Edit: There are 97 couchsurfing members in Missoula.