some of them run on a modified linux distro that Asus customized. If this is what is on there, chances are that you are better off with leaving it alone.
And I suggest linux far above windows on netbooks. Windows 7 starter especially should never be put on one. With 2GB of RAM and a Dual core Atom, it is laggy as hell. Vista is heaver than 7, so don't even think of that.
XP, may go on it. It will be not as secure as Linux, and may give troubles with drivers. But that is what you get with XP.
As for what it cost, depends on where you take it. I know star city charged $25 + the cost of windows to install. I personally charge $10 to install an OS, that is + cost of OS of coarse. Add $5 for partitioning beyond what is required for OS to operate.