With every new game I see on the iPad that piques my interest, I go straight down to the 'in-app purchases' section. If it's got items like "10 gems", "100 gems", "a bucket of gems", "a room full of gems" etc range from £0.79 to £59 or something, I just don't download the game. I assume that it's going to have the sort of gameplay that means if you want to progress, you need to keep buying stuff.
Some games have options like 'first 10 levels free' and you can unlock for a fixed fee, or even an annual or monthly fee (if it's small), but I just think that any game that forces you to keep purchasing larger and larger items is being greedy.