A Bag of Money

So sometimes, as I'm brainstorming things for a script, I like to post questions here, because the more hands the merrier.

These are today's pressing questions:

A) How much money can one plausibly fit in a large duffel bag? It doesn't have to be $20 bills, it could be $100s. If I said that there was $580,000 in the bag, would that seem unrealistic? How heavy would $580,000 in $100 bills be? (I know there's a lot of arcane knowledge out there that will save me the need to actually try to cram 5800 bills in a bag).

B) What's an interesting source for $580,000 in a duffel bag? I want it to be something different from money from a drug deal or cash from a bank robbery, though it should definitely be something shady. It should also be something simple that doesn't require a lot of huge backstory. In basic terms, some of my supporting characters are looking for a duffel bag of cash that disappeared when one of their cohorts was killed.

Let fly.