Let's look at this another way. As a practical matter, it is unlikely that a team is going to get any where near their regulated cost limits from buying aluminum.
You can go to a site like
http://www.metalsdepot.com/
and figure out the cost of metal. Then calculate the amount that you actually used on the robot on a per item basis. That is from rule <R51>
So if you go buy some titanium off of ebay for 10 bucks and the material is worth 250, but you only used a fifth of the material, then your cost basis it 50 bucks, even though it cost you only 10 bucks.
If you manage to use 3,500 bucks worth of titanium on a robot I'd like to see a picture of that.
My $ 0.0001
Ed