Anyone have experience selling a license to their codebase?

I have been approached by a business interested in licensing the code to my app. Without getting into too much detail, they have an app in an adjacent space that would benefit from a lot of what I’ve built, so it does make sense that they’d be interested.

I have a phone call with them in a couple days, and I’m trying to figure out the right ballpark amount to ask for. I’m pretty new to this kind of negotiation.

• If I consider my time valued at the rate I earned at my last salaried job, I would expect I’ve put about $500k of my time into building the app.
• If I consider the current revenue I get from the app, it’s about $30k/year (and growing). So, if I think about the potential money earned from an app like this over, say, the next 10 years, that puts me in the $500k ballpark as well.

That said, (1) I know for client work one would typically charge more than their salaried rate, which makes me think the number could be low, (2) they’re getting the app right away, without having to put years into building it, but (3) they’re not purchasing the business or existing app users, and I will be able to continue selling my app. So this number may be high.

So I’m inclined to ask for something in the $500k range, but be willing to negotiate down from there. Is this way off base? Does more of the value come from the code itself, or the business and users around it?

In the past I think I’ve undervalued my work, so I don’t want to do that. But I also want to offer a fair deal, and don’t want them to balk at the price.

Any advice?