Man I looked into this a bit ago when they stopped giving free Office365 for casuals. The best value is the $99/yr family plan for 6 people. The official cost to buy is a $299 which seems too high. I never understood all the $10 keys on ebay/groupon. I don't understand what you're buying. I refused to pay for any of it and bought a $6000 Macbook that comes with office suite for free!
I've considered just using Google Docs for free, but then I'd have to learn something new. My kids use Chromebooks for school and those things drive me nuts. I imagine the grey market keys come from laptop bundles that never got sold. There is a risk they wont work, but for $15 it's not a big risk.
I bought one of those and it worked initially for a short time, but now it says I have to connect it to a corporate network to keep it working. I wondered if there was a place that had keys that weren't going to give me this problem.