Mods or the modified versions of various games and applications are available free of cost and as purchasable versions. It depends on the service provider from whom you choose to buy the mod applications. Most of the mod apps are free and they might be having small and minute changes to the original version to several major add-on features. But still, there are mod apps that are developed and distributed by major gaming studios and companies who may add extremely novel changes and features to the actual apps and expect the downloader to pay a price for them. These are known as the premium mods for which you have to spend a significant amount of money, and the money that is spent on these mods is transferred straight to the creator of that particular mod. If you are not looking for such premium mods, you can always go for the ones that are free of cost and are available online. When you buy a free mod, make sure that you are buying it from a service provider who has a good reputation among the gaming community, to avoid any kind of scams.