I haven't seen any hidden costs as yet.
At work I'm running a Dell Optiplex 745 with Core 2 running @ 2.13GHz and 2GB RAM.
It runs Windows 7 just fine.
It runs also runs 1 XP virtualisation session just fine at the same time as running Code::Blocks, IntelliJ, Firefox, Excel and maybe a couple of other applications. After that it gets a little slow and stuttery.
I like the user-interface more now I'm used to it, and it's been lovely and stable. I do use XYPlorer rather than Windows Explorer though because the new Win Explorer interface really does my head in.
Can't say I've noticed any hidden costs at all. We checked the compatibility of my PC and it was OK (4.1 units of compatibility, whatever that's supposed to mean), I installed Windows 7 and applications on it (took half a day) and that was it.
I guess if you've an older computer, or one with less grunt, then upgrading will be a pain in the arse because you'll need new hardware; if you bought your computer in the last year or two though and didn't go for totally budget model I can't see what (other than affording the upgrade) the problem is.