Even a cheap laser engraver can do fine work on anodized aluminum (including AR Lowers). This brings custom work prices down by a good bit. If you have a high-resolution .jpeg file of what you want engraved, it's fairly easy for a guy with a laser. Also, if you make an appointment and take the stripped lower in anyone (including those without FFLs) can do it. Many won't, as they'll be afraid to, but I have spoken directly with the ATF over the matter and received assurances that as long as the owner stays with the rifle then no FFL is needed to laser engrave a logo on a lower.
Prices will run anywhere from $60.00 to $120.00. Anyone in the low end of that range and I'd ask to see pictures of their work.
For rotary, or true engraving, with paint fill... well prices will be more. By a good bit.