Sonic & SEGA All-Stars Racing Transformed at 50% off (24-hour deal)
On Amazon, Sonic & SEGA All-Stars Racing Transformed has been returned to the 75% off discount it was at during the first few days of the sale. It's also a 24 hour deal, since the Amazon sale ends tomorrow. Unless you're unable to buy from Amazon, the GMG deal isn't much of a deal.
However, Greenman Gaming is a good place to get the DLC, since you can get 20% off. That's what I did, bought the game from Amazon two weeks ago when it was 75% off, and picked up the DLC on GMG. To say it's not big on content would be an understatement, but Metal Sonic is nice to have, and the Outrun track is really great.