Cheaper Than Dirt has them for 39.95, but you still have to pay shipping.
http://www.cheaperthandirt.com/ARR007-7.html
Edited to add... Here's the M&A 6 position "mil spec" one ($55 plus shipping):
http://www.m-aparts.com/productsDetail.asp?id=1072
I've got one of those, but I also have a couple of the less expensive ones. Unless you're gonna put one of the latest uber-tactical-wannabe stocks on there, you really don't need the "mil spec" tube. The only difference I can tell is that the tube is a _very_slightly_ bigger outer diameter. So slightly, that you can still slide the smaller cheap stocks onto the tube if you want to, though the fit is tight. Of course, if you were to put a stock made for the larger tube on the smaller one, it would probably rattle around a bit. Just be a loose fit.
Unless you're just into spending money to feel extra cool and superior, buy the cheaper one. I'm sure someone will come along with a three page scientific report on why I'm wrong, but they're welcome to their opinions.
Edited to add: Too bad Sportsmans Guide isn't still running the deal they had not too long ago. They were selling them for $25 each.