This brings a compromise to mind. Over the years I've noticed something about equipment rentals: Under normal circumstances it seems by the time you're done, you've paid about a tenth of the price of the thing you're renting.

Why not offer to provide a copy of the source code to the client for one tenth the cost (at your retail programming rate) of that 1500 hours? Just disclosing that you're offering it at 10% of its cost, then quoting them the staggering sum that would be, would make the point that no matter what the client thinks, there is REALLY valuable stuff in there and you cannot simply give it away.

This would be done with a contract that restricts them to use only in the one system they now have. While this, in itself, really guarantees nothing, it may bias the customer in favor of you doing the further work. (Of course, now it's up to you not to be a jerk, nor to lose the files yourself, etc etc.)