I think its ~100k to fill the pool, though I do remember its a reduced amount for the first few times you do it.
So you're halfway around, you could do it with 30 epics for 3 20k sacs for the first one, and you might be able to squeeze out a second one if you get some lucky rolls.
Doubtful you'd be able to get a third - you'd have to get extremely lucky with the 5* drops as well as your sac rewards.
As for whether or not its worth it? Its possible you'd hit low numbers on the alchemy frags and miss out. Would that result be OK? That you're just that much closer to the Alchemist, but no tals.