I don't think making bag of figure is so expensive.
All off them would have been done for disapering, so ?
Running off a series of plastic miniatures is a fairly expensive process, from what I understand. (Especially pre-paints.) It may be true that making the mould is the most expensive element, and those are already done, but the material still isn't cheap. Plastic is an extract from crude oil, after all, and one look at the gas pumps should tell you oil ain't cheap.
When the minis are packaged in a board game, the cost gets absorbed into the whole product. Paying $80-$100 for a gigantic board game may not seem like much when there's all these cardboard fiddly bits to go along with the plastic elements, but that doesn't mean the cost is evenly divided among the components.
Thus, when minis are packaged individually, they seem more expensive than they "should be," when really the price is right about spot on.
The Arkham Investigators are nearly $100.00 for a set of 48 figures (pre-painted, mind you.) There were a total of 38 heroes in Descent and expansions.
Plus 6 from DQ
Plus 4 promos
Plus 6 from Zanaga (assuming they make the set truly complete)
That's 54 figures. Now, I don't know exactly how much pre-painting adds to the production costs, but you're probably still looking at $60-$80 for a full set of unpainted 1e heroes. And that's not even counting the LTs.