Jiraiya is a legendary Sannin, so he must have made legendary money. Also he's a book author.
So why does he keep taking Naruto's money?
Well Jiraiya is a Legendary Sannin alright but there is no explanation given by Kishi in the manga as far as I could remember relating to that attribute. given the fact he is clumsy, show-off (from his entrances in all the battles he faced), an Open Pervert (as claimed by himself).and most importantly he is Naruto's Godfather I think he would have wanted for Naruto to work for the ninjutsu he teaches him (Even though he is his favorite disciple he would've liked naruto to do the extra hard work and may be he could've been messing with him totally because he is his mentor).
Kishi himself said that the best teacher student bonding he loved writing was with Jiraiya and Naruto and Jiraiya's cheap tricks when it comes to money seems to me is more of messing with Naruto in a playful sense more than anything.