I picked up 3 40rd Chinese 223 AK mags today for $15 each. They are made exactly like the 30 rd ones, but of course longer. How rare are these and what are they woth in nice condition.

Worth alot more than $15 each.

Honestly, I don't know, but Norinco .223 30rd mags go for $25-30 or so, so you figure the 40s ought to go for a little bit more.  Easily the $25-30 that a 30rd goes for.

I'd hesitate to call the 30rd mags rare, but I'd probably call the 40rd mags rare.  I have no clue, and probably no one really knows, how many came in the US.  

Needless to say, I think you did good, even if I don't have any supporting evidence.

Ross