The same cost as the first license. I think you can start to get price-breaks at 5 licenses for a site, but if you can figure out who sells these, more power too you.
You may already konw, but you can install vegas on another computer, just can't run vegas on them at the same time.
(it was like that with 5 not sure about 6)
Interesting thread a few days ago which seemed to indicate that now that I own Vegas 4 and Vegas 6, when Vegas 7 comes out I will be able to upgrade Vegas 4 to Vegas7 AND Vegas 6 to Vegas 7, thus having two copies of 7 at the upgrade price (if the same upgrade scheme is in effect then).
I know that Vegas 4 was an upgrade for me so I was hoping I had Vegas 3 and could upgrade 6 again from that, but it appears I upgraded from the lesser program (Video Factory) a couple years ago and never had Vegas 3.