I think in general good developers are going to try to give you something for your money, or they'll be pretty up-front that what they're offering is insubstantial and you'd be purchasing it to support the developer. Bad developers are going to try to con you into buying next to nothing over and over.
Even worse developers are going to convert your paid app to a freemium app
and then make you pay to unlock the full version again.
Yeah, I got burned by that. I got some puzzle game awhile ago that was a "Premium" version that cost money....then they changed it to free and made it an in app purchase to to unlock the full version. So, when I got the new iPad and was redownloading all my purchases, it wanted me to pay $$ again to unlock the full game, which I had already bought!!!! Total BS.