Been to Stagecoach Inn, Westside Drive-In (on Fridays and Saturdays only), and Lock Stock & Barrel. Of the three, I thought LS&B had the best. But for the best value, and if you don't mind not having a fancy sitdown meal, then I think Westside is the way to go. $15 gets you a cut of rib plus salad, potato, and roll. Once in awhile in the Friday Scene section of the Statesman, they'll have a $2 off coupon for that weekend knocking it down to $13. The salad is cheap and horrible, but the potato is pretty good. And while the cuts of rib have been a little fattier than I'd like, the taste is decent, and they've been cooked exactly to my requests. Go on Youtube and search for when Diners, Drive-Ins & Dives visited there. They talk about what's put on the rub that is used on the rib, and that's what eventually convinced me to try their prime rib.
Stagecoach Inn Restaurant & Lounge
3132 W Chinden Blvd, Garden City, ID 83714
Lock Stock & Barrel
1100 W Jefferson St, Boise, ID 83702
1939 W State St, Boise, ID 83702
I was at Cottonwood Grille last weekend choosing a menu for an event in August. They had prime rib on their menu, and while I cannot vouch for it personally I have always had good experiences there. I hope this helps a little, if not sorry!
913 W. River Street, Boise, ID 83702