Nope. If you want to shoot economical .30's, there are none.
.308 isn't readily found surplus, most is used in MG's and there aren't any hunting rounds adopted by NATO. To get those off the shelf you have to pay the going rate of $1 a round.
.300BO isn't military issue anywhere, no surplus at all, right back to the store shelf, same prices.
Which means to get either cheap you set up your mountain top retreat and sit at a bench reloading to get it.
If you stick to 5.56, will you find cheap ammo? Sure - but just like those above, no hunting round cheap as NATO 5.56 and the US doesn't issue them in bulk. Back to the store shelf.
It's the FMJ plinking ammo for practice that GI surplus excels at, and why 3Gun settled on it as many competitors can pick it up and use it, being 3Gun is a skill of fast acquisition and hits on torso targets, not 1" bullseyes. The issue 2MOA ammo is plenty accurate being used for it's actual intended target size.
Cheap plinking ammo is why 7.62x39 is so popular, too.
At one time there were numerous complaints about one certain cartridge being "pricey." It was as if a campaign was spreading across the internet to trash the round because there was no cheap surplus to shoot up.
Lately I see a lot about how .300BO is "affordable," when in reality standing at the ammo shelf I see the same prices - except for XM surplus 5.56 or import x39.
Since it's about hunting, for the most part it's NOT about cheap FMJ plinker ammo, so we are standing together looking at boxes of ammo side by side and choosing what we see - or, reloading. Hunting bullets aren't cheaper than FMJ sitting on the bench either.
Do the math - you don't pick an alternate cartridge for "cheap," because it won't be. You pick it because it has the ballistics you need shooting it. About the only time cheap comes into play is when you shoot paper and make a lot of little holes for fun. Shooting game, the ammo isn't cheap - but we don't plan on blowing away hundreds of rounds of that into a gravel berm, either.
Hunting ammo in bulk isn't a big deal except for contract hog hunters. The rest of us rarely use more than two or three boxes of expanding point hunting rounds a season.