I have never shot cast rifle bullets though an auto loader. Midsouth Shooters Supply has some bulk hard cast rifle bullets
Berrys has 150 gr plated flatpoints, of course those cost a bit more.
If you use jacketed, just Google 147 gr FMJ, you should find those for $10 per hundred.
.309" is an odd diameter if you're looking for jacketed bullets. The BO uses a .308" jacketed bullet. .309" would be a cast bullet like Marco Califo talks about.
"...should find those for $10 per hundred..." Not any more. Searching for '147 gr FMJ' churns out hordes of 9mm bullets too. Add .308" and you get 147 grain milsurp rifle bullets(use 150 grain data), but not at $10 per 100. .30 cal FMJ's are getting scarce too. Military doesn't use 'em as much anymore. Primarily MG's only now.
Widener's is showing 500 IMI FMJBT's at $79.95. Buying bullets on-line gets expensive quickly too. Shipping costs.
309 cast are $10 per hundred. Magnus Bullets will sell & ship direct as well as sell through Midsouth. They are in the $40-50 range for 500. You pay shippung at either. Magnus uses USPS flatrate boxes and can ship several boxes in one. Midsouth shipping costs more.
The 150 gr Hornady FMJ are now running $20 per hundred, but I know I have paid less before.
Assuming you want jacketed .308 bullets, Wideners has some IMI 147's for under %0.16 each. Just don't expect too much by way of accuracy from NATO ball bullets.
For light bullets the .308 110 grain are pretty cheap. I have used Hornady, and Remington in .30 carbine. I load them in 7.62x54 for my wife to shoot her Mosin. They make good gallery loads. They worked ok for light .30-30 Win loads. They hit minute of steel plate at 25 yards. (6 inch plates).
A friend of mine just got a .300 BO upper for his AR and is waiting for brass. He plans to try the light bullets first for the heck of it, and due to the fact that he has them on hand.