Recommend NOT doing this. It's against Google's TOS and you can quickly find your account banned and/or site de-listed.
Google Vouchers are intended for people NEW to Google Adwords. It used to be that the vouchers were setup as one per ID, but they are now setup as one per domain. Google detects the landing page being used. If you have multiple accounts with the same landing domain, you will be banned.
Google, at least in October & November 2011, was generous enough to provide people warnings - but from what I've read on the various black hat forums, the warnings are now very few and instead they're being blocked/deleted in bulk these days.
Any attempt to game the system for $75 or $100 is not worth the risk. I'm all for coupons and discounts, but that's not what this is - this is exploiting the system. Google now manually reviews all adword account signups and because of the actions of people setting up multiple adwords accounts, Google now manually reviews adwords ads/campaigns prior to going live.
You're taking a risk - don't do it - you will be caught if you try to game the system.