I have noticed that the Saggitarius's Quest are quite expensive. Is the prize worth it? O.o

It's worth it, but only if you spend a limit of 20k per item. If you spend more it's not worth it most of the time :/ SO just make sure not to spend more then 20k on an item and it should be okay ^^

From my own experiences I usually get 15-20k for completing one item quests, 30-60k for two item quests, 80-120k for three/four item quests.

I pretty much only spend 20k per item and I always make a profit. (So 40k for two item quests, 60k for three item quests etc.) You'll probably have to skip a lot of his quests as they are expensive, but if you be selective you can make a pretty nice profit. Good luck! :)

i tend to earn alot on Saggi, though as weedle said you need to be selective on it and not give in to everything he asks for xD My limit is usually 20k for 1 item and not more than 40k on 2-3-4 items I'm so cheap i know xD

It's worth it, but you have to pick and chose your quests. I wouldn't spend more than 20k if he asks for 1 item, no more than 40k on two items, no more than 50k or 60k on three items (depending on how well he was paying out on prior quests) and I'd say on 4 items it depends. On bad days I won't go over 60k, but if he's paying out well I'll pay more. Once I spent a total of about 80-90k and he gave me 200k so it really depends.