A company called MGI is about to release a lower/upper combination that lets you use AR mags and barrel or ak mags and barrel-but I digress
A guy I was shooting with over the summer said he uses his 7.62x39 upper for hunting whitetails here in upstate NY. I've never owned a 7.62x39 upper, but have heard that the bolts are not the greatest, reliability wise.
We constantly argue here about hunting deer with .223. Lots of guys have done it, personally I wouldn't, but we generally have bigger deer up here.
I don't know that you're really saving money over a 7.62x39, but there are some larger caliber uppers that are sort of in the 45-70 range. I've shot a 50beowulf and it feels like a 20 gauge slug gun. Rock River Arms is going to start selling the 458, which I believe is similar. Olympic sells 243 wssm uppers, but I have never seen one.