They seem more of a novelty to me, unless of course you had a mad stash of AK mags and almost no AR mags. After all, isn't 5.56 a better round balistically than the 7.62? And aside from cheap Russian stuff (which is supposidly harder and harder to find right now) aren't the prices fairly close in price?
All you can do is contact some of the big wig dealers in California that are doing the lowers and find out. Just keep in mind it's a oddity, so it will command a higher price than those that are just mass ordered.
ETA: I think I heard somewhere that Atlantic REFUSES to take their rifles apart, and will NOT sell stripped receivers. That means you would have to buy the WHOLE rifle and find either an AW FFL that would mod it to be legal, or find an out of state seller who would do that, or strip it and send only the receiver to the FFL and all other parts to you at which point you would have to do the last leg of the work to make it legal.
I think its a great idea, as its next to impossible to buy a new 7.62x39 upper or AR mags in 7.62. Its not round that is ballisticly inferior, its usually the rifle spitting it out. AK's, the leading spitter of 7.62x39, are not precision rifles. Therefore you do not get a tack driver, and the market responded with cheap ammo, perfect for non-tack-driving rifles. My Russian SKS can shoot 3-4MOA out to 150yds with cheap russian ammo. But you can buy good ammo or load it yourself. BTW, many people hunt with 7.62x39 and find it quite effective against deer and other medium game.
On the other hand, I seriously doubt that Atlantic firearms will ship you the lower alone. To make it Kali legal, you could put a set screw into the mag release paddle to prevent it from moving. Along with a 10 round mag, once the mag is fixed you could bring it into the state. you need to find a person to do this, and that person would need to have a 01 FFL (otherwise could be seen as straw purchase).