The latest edition of Game Maker is "Game Maker 7". Game Maker 8 is currently in a beta stage.
Pro and Lite editions are available. Pro costs £20 or $25 (yes, the exchange rate does not meet the actual seen on xe.com); significantly less than other engines but it should be noted the Pro version is not a necessity, it just includes some more features which can be useful such as network play dll compatability and sprite rotations, etc.
GM Pro is notoriously hard to pirate and find good cracks for; most available are viruses so I wouldn't recommend cracking it (not that anyone here would).
You purchase Pro inside the editor. Note that currently UK Maestro cards do not seem to work, but you can pay by various credit cards and Paypal.