I don't think AJ was ever even implied to be poor, most likely same for Rarity bein' rich.

Define rich. Rarity owns her own business sure, but she's never really shown to be wealthy.

She might have a lot of jewels, but their actual monetary worth is kind of ambiguous, especially since Spike just eats them all the time.

And we aren't fully certain how much of their diet consists of Apples. They seem to eat different kinds of grasses and flowers more than anything, and sugar-stuffs, can't forget their sweet tooth.