At lv21 (5040 growth), using great fruits becomes fully efficient money-wise, since you need 2 normal fruits + a stack of ratz (29 rum), equivalent to the price of a Great fruit. Lvling above that with Normal fruits actually results in money loss.
It is, however, worth using great fruits long before that, simply because you save a ridiculous amount of time, especially on 2nd gen pets. My personal preference is starting to use great fruits at around lv10, where jumping that one level becomes more and more efficient.