Side Project

You should also consider buyer's ROI, if it takes to long he/she would not consider buying it. Other than that you should calculate the price related to complexity, features and uniqueness of your project. Don't compare it to the prices of business applications that offer something usual. I hope this helps a bit.

Software is expensive. Start at $50k and work your way down from there.

The project sounds relatively unique (not many like this are being sold), and you completed it in 2 months, then the alternative for the buyer is to hire someone else to complete it in two months. With $100k average annual engineering salary that would take them $17k. If you add cost of hiring and freelancer premium, it might be $25k. And that would take a risk of the project not getting completed, or ending up just poorly done, plus just waiting additional ~2-3 months. So it would make sense for them to buy a ready & working thing for $30k if they are not in a hurry and for $60k+ if they are. These numbers go down proportionally if engineer salaries are lower around the buyer

That said, I agree you should just give a starting number and state that you’re open to negotiate. But I think you should start with $30k or even $60k if you get a signal that the buyer can afford it, is surrounded by highly paid engineers and has a good idea of what they are going to get from it (as opposed to casually asking)