The mag risers are not as collectable or as valuable as the wood risers from the 70s. However, they are reliable and they shoot well. I have two A-mag takedowns and I like them.
Yours should be worth around $300 give or take. But, as with anything, a it depends on condition, draw weight, who is in the buying market at the time, etc.
Make sure you only use a B-50 dacron string on that bow. The string should be 4 inches less than the AMO length.
P.S. If you want to find a new home for that B-mag, let me know. I'll take it off your hands. I am certain there are lots of other archers out there who would do the same (but I am first in line).
Best regards,
Joe