$135 for a stripped lower is high, but then again, that's sitting there in your gun store ready to go, and that makes it more reasonable.
Usually I note that the "big name" lowers (lower receivers made by companies known for their complete guns) tend to go for more than those who don't make guns. A Rock River, Stag, S&W, or other branded lower will usually cost more than an Aero Precision, Superior Arms, Spike's, or other "lesser" branded lower... even though they're often made by the same companies (Aero Precision makes them for many other companies).
Generally, $80 is more a target price for a lower, but then you have to factor in shipping and transfer fees... getting the price up closer to the $135 price tag in your local shop.
$135 for a Stag lower without further fees, that's a reasonable deal. If you want to build your own rifle it will be a good way to start.