Scholastic / Journey Ed is advertising a a $39.98 deal for a Windows 7 Pro Upgrade version. You have to be a qualified educator or student. There is a verification procedure that requires you to send some information by email or fax before they fulfill your order. You can purchas a 64 bit or 32 bit version (you have to poke around the site a bit to find the $39.98 price for both flavors). Keep in mind that it is the Pro ( so you can log in to a domain) rather than the Home version and it is an Upgrade version.
Many thanks to two great Connecticut tech teachers – The Brothers Hendricks (Jamie and Richard) for passing this link along.