[anothertraveler]

If you study a bit of corporate finance, you can learn the mechanics of valuation. At the end of the day, though, how do you determine the market value of any asset? You auction it off. If you keep it, you are saying it is worth more than $50K. If you sell it, you are agreeing that the market value is whatever the other party is offering you.

Rather than trying to calculate explicitly what it's worth, have you considered trying to find other buyers?

Edit: Another thing to consider is, if this isn't a life changing amount of money to you, you could treat this like an empirical experiment to learn the process of selling your company. This will be invaluable experience in your next start up/side project, or for any future business endeavor where you engage as an owner or potential owner of a company in a transaction like this.