I wasn't saying for sure that 6000 series is more expensive than 7000 series, I was just trying to speculate as to why different alloys have different costs. I don't know what aluminum prices are, but there are a lot of factors... manufacturing processes are just part of it. Just like there is straight gauge 4130, butted 4130, Reynolds 520, etc, there are many different types of 6061 or 7005 aluminum tubing you can get. I suppose you can't really say that 6061 is "nicer" than 7005 or vice versa, and you can't say the same about costs either. I'm sure you can find both "cheap" and "expensive" frames with all sorts of aluminum alloys.

There's a lot of reading online and on BF about the differences. Here's a thread, for example:

http://www.bikeforums.net/archive/in.../t-482375.html

I'm sure there are other reasons for choosing an aluminum alloy over another one. Machineability, weldability, heat treatability, ultimate tensile strength, stress/cycle data, etc. I don't know why Bianchi uses 7005 and Tiemeyer uses 6061, but like I said before, you probably won't notice the difference. The main benefit of 7005 is that is has a higher UTS and thinner tubing can be used, but I think with a proper frame design you can get an equally stiff 6061 frame without compromising the weight.