Hmm, for Baby Ribbs, maybe $10 to $13? He sounds really nice, though, so you might be able to get more. A mint Twinkles might be $5 to $8. The rest I'm not too familiar with.
As far as buying, for a nice Fizzy, I'd guess $13 to $20, though as she's common you might be able to find a better deal. As far as the rainbow curl ponies, most would go from $12ish to $25 individually? I think? in nice condition (i.e., with reasonably nice curls and no fading). You could probably get those guys cheaper if you bought fixer-uppers who still had silky hair and just needed curling.
Hope that helps!