This is presumably a limited time offer, but you can currently upgrade from the free version of game maker to the standard version at no cost, or to the professional edition for US$50.
The process is a little tricky, and not helped by slightly clunky UI:
- Download and install the free version of Game Maker.
- Select the beta updates channel, and upgrade to the latest version.
- Run the software, and a dialog will display the offer.
- You'll need to enter your email address (I had to do so twice) and register an account to be sent a licence key.
- Enter the licence key and you're good to go!
The terms and conditions say you have to agree to receive emails from YoYo Games; I'm not sure how frequent these are or what the content might be, but I'm sure you could un-subscribe or filter them if it's a problem.
Just thought I'd share for anyone who might be interested but doesn't want to outlay the normal cost -- the standard version is still reasonably minimal but much more capable than the resource-limited free version. :)