I usually pay about $20 to $30 for a decent 21" rim, maybe a little more for a real nice one. There are so many problems with model A rims, especially the 21" ones - excessive rust pitting, flat areas on the outer part indicating the rim was run without a tire, separated or badly bent spokes (slightly bent spokes can be fixed), cracked center part (almost always where a long spoke is), wallowed out lug holes, rusted outer part, and excessive run-out. If you can take a lug nut to check the 5 holes and if you can mount the rim on a spindle and spin it, that can help insure you are buying good rims, beyond the visual inspection.
Rusty Nelson