I've seen a fair amount of CO/UT/WY/MT/OR/WA, not so much in Cali. Below are towns as opposed to the mini-cities of Boulder, Bozeman, Bend, Ogden, Missoula, Santa Fe, Flagstaff. Great places, but not mountain towns.
semi-cheaply?
Paonia, CO
Dillon, MT
Leavenworth, WA
Twisp/Winthrop/Mazama, WA
Livingstone, MT
Cascade, ID
Lander, WY
Not as cheap:
Gunnison, CO
Ridgeway, CO
Glenwood Springs, CO
McCall, ID
Whitefish, MT
Salida, CO
Buena Vista, CO
Not Cheap:
Carbondale, CO
Eagle, CO
Fraiser/Granby, CO
Durango, CO (borderline mini-city)
Victor/Driggs, ID (zombie lots are selling)
Taos, NM
Those places are still cheap in comparison to my town: "The 2015 average Aspen single family home price per sq ft increased 25% in the past year to $1,311 sq ft. Aspen condo price per sq ft increased 2% in 2015 to $1, 201 sq ft."
Someone mentioned Estes Park earlier - shoot me before I live there. Have you seen the mini-van hordes in the summer???
Down the highway is the aforementioned Nederland. Nice idea, but the concept is played out: true weirdos who showed up decades ago and a bunch of pseudo-intellectuals who "don't live in Boulder."