Lou,
I fly to MT every summer from ATL. Historically, I get the best deal flying into Billings vs. Bozeman. I don’t seem to get BZN on sale anymore. I bought ATL – BIL last week for $277 plus junk fees on Northwest. That sale has been taken off the website. I booked through their site http://www.nwa.com.
I also use a really neat airfare search engine called http://www.sidestep.com . It searches most major airlines as well as Orbitz and a few others. Try to travel on a Saturday for the best deals. Sidestep lists the following on Northwest for July/August departures.
TYS-BIL for $519 plus junk
TYS-BZN for $522 plus junk
TYS-BZN for $534 plus junk
Hope this helps.