Depending on the quality of the "name brand" on the stripped lower, I would say $100-$160. Of course yesterday's little ripple through the waves of the worried is likely to change that.
The important thing to remember is that the Lower Receiver is all you need to get on the books, and in the grand scheme of what you are building, it's going to make but 3%-5% in a quality build who's name is on the outside.
Granted, there are those that will tell you that if you don't have that NAME BRAND, CNC cut, spec'd designed model it's going to freeze up on you at the worst possible moment and leave you and your family in a pile of blood and brains, but I don't subscribe to that philosophy.
Your lower is merely a platform - from there you pick the parts that go in it and you make it what it will become.
Stay away from anything made of pot metal, usually outside the country. Buy something that is Aluminum and feels good in your hand, doesn't have chatter marks on the machine cuts and doesn't feel rough and uneven as you run your hands long the edges. If you can test fit a mag into the mag well, so much the better.
As for the upper, you want a nice fit between upper and lower, and you want one that is going to match up well. Don't buy a Double Star lower ( small pin set ) and find a great deal on a Colt Upper ( large Pin Set ) and expect them to go well together.
Get what you consider a good deal Gus. What you can afford and what you are "willing" to pay. Once you have that, I can show you some tricks to get some extra accuracy/performance out of even the "Tier 1" stuff out there with a little know how and a little elbow grease.
Between myself and a few others, like Slo, we can get you a good weapon platform, built by you, that will keep the Space Zombies well outside the wire.
PS: Check out Slo's Link above. They have a CMMG stripped lower for like $113 that will mate up with damn near any upper out there and is from a good, quality manufacturing house. You might not even need to go to the show if you know a good FLL near you....