Cheaper to buy factory or reload?
Do you already reload for other metallic cartridges? If so, then adding a set of dies and some components is a pretty straighforward endeavor. I can load 220gr subsonics for about $9-10 a box (20 rounds), about half the price of cheap ammo, and 1/3 the price of the more expensive, subsonic Blackout ammo.
If you're not already reloading, then justfying a whole loading set-up just to load .300 Blackout ammo seems less ideal. I suppose it depends on how much ammo you plan to shoot. Potentially saving $20 a box on ammo would pay for reloading equipment pretty quickly, if you shoot a lot.
Looking for the easy button. Thinking 220 Sierra's to stay subsonic
I bought some 220gr BTHP factory seconds from Midway about a month back. They were around $30-35 a hundred, with free shipping, which is pretty cheap these days. They are now gone, at Midway, but MidSouth still has some:
Link.
I like Lil' Gun powder in my Ruger American bolt action. The smaller powder charges with Lil' Gun make for noticeably quieter ammo. I don't have an AR chambered for the Blackout, but my friends that do, prefer CFE BLK and/or AA1680 powders for their ARs. One friend said it took some experimenting to arrive at a load that reliably cycles in his AR, and once he finally got there, he doesn't want to deviate from his AA1680 load.