If we're going to use eBay as a reference (which I don't think is unreasonable...you can look at completed items and get a fair idea of what things are actually selling for), then it goes about like this:
P1 Magnet $500-800
P2 Magnet $25-35 (C-shape)
P3 Magnet $20-25 (D-shape, also works for P2)
P4 Magnet $15-20 (also fits Ladies' Models)
Don't forget Dress Keychain Magnet $50-100
Generic Magnet envelope $5-10 Keychain envelope $10-25 (rare!)
Range allows for variations in condition. Nothing here is set in stone, of course; items might go high or low outside these ranges depending on buyer's need or general bidder apathy...bargains still do show up on eBay, and newbies occasionally push prices beyond any limit of reason..