The ATI Omni is a poly lower. Complete lowers run less than $150 and have a metal parts kit. I bought one, and have been pretty happy with it.
A stripped aluminum lower and build kit will cost more than a complete poly, but will be of higher quality if you know how to build it. Depending on your intended use, you might want the sturdier aluminum. On the other hand, the poly might suit your needs just fine.