http://www.davesmetalworks.com/m4.html
I got one here for $75 which seemed like a great deal to me. I'm not advertising for them. I just got my first M4 and have been going through the parts. I couldn't handle spending $300 for a titanium tube. The steel tube is going to be around 45% lighter but what is a couple of ounces? Maybe I'll change my tune later and wish I made a different decision but for the price, I felt like it was the way to go... Also, almost everything is out of stock and this one I got in just a few days.