Black market prices are copy paste from cn, since hokage minato was considered a real rare back in a day his frag price in black market was 1000cp/frag. EN servers had him given away left and right as jackpots (infamous minato shisui meta), then redeemables and his price nowadays oscilates around 120-150cp/frag. Either way I wouldnt use black market as a price reference, since we often had answers from mods that ninja in X event is better/worse deal than in Y to make more variety, while in reality most of those prices are what they are and they have hardly any reflection in what ninja is worth in actual game.

You could think mei would be that expensive for an outdated ninja because of milking proces for skillbooks, but up until now she was virtually unobtainable through events (she might have appeared in 1ingot sales, cant say because i literally never look there). Perhaps somebody didnt plan that right. Would she be usable again with all necessary books (mystery, passive, chase)? Yes. Would she become sudden op? No. Gotta keep waiting for skill experiments to finally be implemented.