At what point are you rich enough to actually benefit from Saggitarius quests, so as to complement them with Old Wizard Quests?

While doing the quests if you fall below 750k you can't turn in the items, so I wouldn't do Saggi's Quests until you have at least 1 million sP. Saggi's quests can give anywhere from 10k - 100k depending on how many items he asks for.