There are several highly rated games that do this. If i remember correctly, "Where's my water?" is one of them. One other great example i recently experienced

There are several highly rated games that do this. If i remember correctly, "Where's my water?" is one of them. One other great example i recently experienced was BADLAND. Its a really cool game in which they give you a level pack called "Day I" for free which has 40 levels. If you want the second set of 40 levels (which are way cooler btw), you have to purchase those. Also, they let you purchase multiplayer levels (6 of those are free btw). So just as you said, they let you demo their game and if you love it, you buy the rest of the game. But one thing i REALLY loved about the devs was that that they you EARN the extra levels. If you unlock a certain achievement, you get the Day II pack for free! So that's like the devs rewarding their biggest fans. (I unlocked those levels for free btw). Disclosure: BADLAND is also powered by video ads. You can pay a little to turn them off. There's also the option of a small fee that's not enough to buy the levels, but it lets you turn off the ads.