I was asked to sell the source code of small utility app I did years ago with existing users of this app. I tried investigating how to put price on the source code and haven't come up with a good solution so far.

I first tried searching the net, but information I found there are somehow far from reality. Then I found a few people how also sold their source code with users as well. But their price seems unrealistic (too high).

For example, one person had an app which price was around $200 for 1 user and he had 80 users. He sold the source with users for $30k. How did he come up with this price?

Is it a good price if I charge the code by formula:

num_of_users x app_price + app_price x num_of_new_users_in_one_year

?

This means that I count the price by selling each user for the price of the app then adding the amount of money I earn in 1 year from this app.

If this is a good formula, what shall I do with sources who do not have users yet?