It's unclear from the context whether OP seriously wants his question to be answered. By which I mean the question he's asking of whoever did him a favour, not the question he's asking EL&U.
Per @EnergyNumbers answer, a common "stock phrase" is "How can I ever repay you?". But note that this is not normally a real question to which an answer is expected - any more than you expect someone to tell you about their health if you greet them with "How do you do?".
If on the other hand OP really does want to know what quid pro quo he can offer in return for the favour, I'm afraid he'll need to use some longer form of words. Preferably his own words, since any commonplace expression with such a meaning would almost immediately be overused and come to be seen as a polite rhetorical question.
Noting OP's reference to "his own culture", I'll just add that in my particular culture, a "favour" is something nice you do for someone without expectation of reward. If there is an expectation of reciprocity, I wouldn't call it a favour – I'd call it a bargain or similar.