Bad News: The $ 249 round-trip fare, BIL to DEN does need to be booked about a month in advance, GTID, and I suspect those dirt- cheap fares may go away during the summer months. The normal fare on very short notice was $ 589.
Good News: On Priceline, I fed BIL to DEN departing Wed., Mar. 19 and returning Fri., Mar. 21, and the best Priceline could do was $ 295 round trip. That's obviously about $50 more than $249, but still really good, especially on 2-3 days notice.
I think if you work your computer and phone a little, GTID, you can get down to Denver and back on United from BIL for about that $295 I found just now on Priceline.
Good luck and if you're flying out of BIL GTID, breakfast, lunch or dinner in Downtown Billings for you, you and the wife, significant other, suga boogar, etc. is on me !!!
Go GRIZ !!!