This is my take. You are dealing with a pro tenant. They know the law and how long they will be able to stay rent free. They know what it costs to rent for the time they will live rent free. They also know what it will do to their drug dealing business.
Personally, I don't think your offer is all that generous. Think of it this way they still owe you money they have no intention of ever paying and will have to go through the brain damage of finding a new place. I think the attitude you got was "What are doing offering me chump change don't you know what this is going to cost you?"
2 ideas. Ask her how much it would take for her to be gone by Monday noon or even Nov 1. Figure out how much the eviction attorney will cost you as well as all the other fees and lost rent. That is your max figure for cash for keys.
Also when you do make her a real offer (your last best price) let her know there will be no other offers, it's off to court do not pass go do not collect nothing if the don't take your best offer.
I'm sure you know this but no keys=no cash. Nothing up front. You might want to rent a storage unit for them. They rent it and you give the storage guy the money for the rent.