What type of 8mm should I start buying in bulk?
What's a good price per round?

You're a few years late on cheap 8mm surplus.  Get whatever you can afford and stack it deep at the 42 eats it up fast.  The best surplus was the Romanian and you can still find some here and there.  Next would be the Yugo.  Then there is the Turkish stuff and most people don't like this in the 42 due to case failure.  But I've put countless crates of the Turkish thru the 42 w/o problems; dates varied from 1930 thru 1955.  Was great cause it was $50 per crate of 1400 in quantity.

And you need to stock up on 8mm barrels as you need to switch them often.  The 8mm barrels have dried up but 308 is still plenty.  So maybe better to run 308 initially as you stock pile 8mm stuff.

I've been wondering this myself, is the Romanian surplus that everyone raves about the 150g stuff?

Mike

Yes, the green steel case stuff in sealed cans.  Runs great in all MGs.