Microsoft Office 2010 Home & Student 3 User $90 at Newegg

So how do they tell that the computers are in/from the same household? Do they keep track of the router's MAC address?

^ They probably stipulate that without any enforcement mechanism, and even if they were to use the MAC address or IP address of the router, that would cause too many invalidations as both are subject to change with either dynamic IP addresses, router mac cloning or replacement of equipment, installation on a laptop roaming with wifi, etc.

I think Dave is correct. Personally i'd rather have an enforcement system but that's just me. A website you can long into to manage product keys, activate & deactivate licenses, etc. Of course this would make it easy to resale old software and be confident you can sell old software and the key will activate. Or offer a transfer fee if you are selling an old Microsoft license. That way the license can be registered under another account.

Open Office is FREE.

$90 is not much more than a Chinese bootleg copy from fleabay, like the one I received recently and am having trouble getting covered on, as the seller wants it back first, and the FBI wants to look at. Pretty good copy if you don't look too hard or have a real one to compare to.
1: The carton is an excellent copy on the proper paperboard, printed beautifully but has numerous typos in the printing on side and bottom.
2: Inner plastic case is a pretty fair copy, but a little sloppy at the edges.
3. Diecut stickers on top and inside of the case a decent copy unless you've seen the real thing.
4: Instruction booklet inside is an obvious laser print, not lithographed and varnished like the real thing.
5: Holograms and printing on the disc are stickers/labels, not imbedded in the disc like authentic Microsoft stuff.
Incidentally, the Key code works and registers.
I've only tried two installs out of the three allegedly allowed, but it would be interesting to see if it would work on more, like an academic license.