Forza Motorsport 6 is currently available at no additional charge on Xbox Games With Gold, but if you want to get every single expansion and car available for the game, you can pick up every last piece of DLC at a massively reduced price too - but make sure you act quickly, because this is a limited time sale.
The Forza Motorsport 6 Complete Add-Ons Collection is currently available at a 95% discount. If you're in the US, that takes the price down from $99.99 to just $4.99, but the same discount is available in all other regions as well. This discount will be available for the next 4 days, so if you're interested, get on it fast. It bundles together every single piece of DLC available for the game, including every single car, track and career mode that was released post-launch.
The Complete Add-Ons Collection includes both major expansions, the NASCAR and Porsche packs, which contain unique campaigns alongside additional tracks and cars. Also included in the Car Pass, over 50 additional car packs, and the VIP pass that doubles your race earnings (and contains even more cars). This is a significant amount of content if you're looking to get the most out of Forza Motorsport 6, and it'll make progression through the game easier. These pieces of DLC are not available at individual discounts, so you'll need to make sure that you specifically purchase the Complete Add-Ons Collection for the discount.
Forza Motorsport 6 was originally released in 2015 and was received warmly. In his 8/10 review, Miguel Concepcion called the game "yet another great reason to get back in the driver's seat." The series is taking a break this year, making 2019 the first year without a new Forza game since 2010.