Some apps, of course, do this. Paper on iOS, for instance, is a free app that relies upon you buying each of the tools. Alternately you can buy the whole set of

Some apps, of course, do this. Paper on iOS, for instance, is a free app that relies upon you buying each of the tools. Alternately you can buy the whole set of tools for a given price. There are official emulators (e.g. Atari) that let you buy individual games, or you can buy the set for a price.