Forged Aluminum and Mil.-Spec. is all you need to look for. After that it becomes a matter of which company's roll-mark you like best. Stag, Armalite and of course Spike's are some of the more popular ones. $85 -$135 out the door is a pretty normal price, but Palmetto State and others have had some great sales recently for around $50 recently.
The parts that go into the lower are more important than the shell of aluminum that holds them.
BTW, aluminum billet lowers are nice, but are very expensive for most people's needs.
Fear is a Reaction..............Courage is a Decision