Land east of Boulder a ways (30 minutes+) can be affordable. Land < 30 minutes from Boulder in the North and East directions can get _very_ expensive. Lots of land west of Boulder in the mountains that can be built on too, but it's also expensive and can be pretty rough regarding roads, fires, floods (as we learned last year), etc....
6 acres of land IN Boulder with one house, water rights, and agricultural zoning has been for sale for a few years for $5m. One of the last parcels like it in the city. Will probably sell soon though for redevelopment.
EDIT: Regarding "cheap" being a subjective term. The neighborhood I rent in is < 10 minutes by bike to downtown and has mostly 1/8th acre single-family-home lots. Recently rebuilt houses with more 2000+ sq. ft. on these lots sell for ~$1m+. There is a lot of money being spent to live in the area.