You'll probably find those two features to be incompatible.

I'm starting to see boxes of Winchester 7.62x51mm (147 gr, FMJ) start to show back up on the market. That's about the "cheapest" I'll go when it comes to my M1A. A lot of the cheap surplus stuff that you used to be able to find was foreign machinegun ammo, delinked and repackaged, and often - corrosive.

When 5.56mm is showing back up on the shelves at 150-200% of what it sold for pre-panic, be prepared for some sticker shock. While bulk 5.56mm has been available then an now, I've never seen "quality" 7.62mm offered for sale in the same bulk quantities. You may need to get used to the idea that you'll buy your ammo one magazine at a time.