You're absolutely right about the costs of running real iron - Windows adds $1,000 per machine. But Windows virtual machines cost the same as Linux: about $20/mo for a lowend VPS. Plus the code is compiled and tends to run more efficiently than Ruby for instance. :-)

If you are awesome and outgrow that VPS with some paying customers, then I think you can swing the $1,000 on a windows license.

I really hate being a shill for MS...