In summer 2020, I had a group of 15ish sites that I'd open every morning to check if they had ANYTHING at any price. Palmetto State is usually pretty good at keeping prices low on ammo, but availability can still be a problem.
Since then, I've used Ammo Squared subscription. They let you set a budget, select what type of ammo, then you can set up shipping triggers or just have them store until you're ready. I've been able to stay below below 40 cents/round for 9mm, and around 55 cents for .223/.556. It has been as low as 32 and 38 cents, respectively. Also, free shipping if the value of your shipment is $250+.
Only drawback is you don't know for sure what brands you'll get. Most of the 9 has been CCI Blazer or PMC Bronze. The has been almost all PMC Bronze with a little American Eagle. The brands for other calibers I get are all over the place. I don't think I've gotten more than 2 boxes of the same type of .22 from them.