BigCo isn't buying you for the $400k per year. Try to figure why they are buying you and work from there. Figure out what your number is and what you think their strategic value of your app is to them. That becomes the floor and ceiling for your negotiations.

The best way to know what you're worth is to engage an investment banker that knows there space, If your app is worth acquiring, there are several other companies that might want to acquire it. A banker's job is to bring those folks to the table and get you the most money. This sounds like a good engagement for a niche bank.