When I bought five new tires and wheels last year for our previous Heartland Trail Runner, the shop that did the axle and spring replacement and repair charged us his cost for the wheels and tires already mounted, but not balanced at $135 each.
As for keeping the old BlowMax tires for a spare . . . here is my take on that.
If you don't trust the BlowMax tires on the road then how can anyone even consider putting them on as a spare?
When I sold the entire set of five wheels and BlowMax tires that came on the Trail Runner I got $150 for the whole set!
Took me about a month to unload them on CraigsList.
First four shots are the old BlowMax tires & wheels, last shot the new set