Well, you can also just get some special licence key which works for any version of the product (from the basic one till ultimate-x64), and it'll work forever no matter how much u use it. They'll be around on the web but never looked for it so not sure how long it takes to get those.
Though you can be sure, those ppl selling licences for 3eur or whatever make 3eur profit on each licence. Ms won't see a single cent of it