The net assets are basically $15,000 worth of IT gear like a laptop and some servers. There is current month to month contracted revenue spread across a diverse base of users paying $5-100/mo each.
How much would you offer me for this company? How much would you take for this company if you currently owned it?
Something is worth what someone else is willing to pay for it. Obviously whatever the company sells for is worth exactly that amount at that moment in time. Valuing such a business at $15k would be absurd, just as valuing it at $15m would be.
Thus, the difference in asset value vs. what you paid is just called "goodwill" - perhaps the name hangs people up a lot.
If you then turn around and sell the same company later for a loss, you take a write down on that goodwill.
Certainly not perfect, but there aren't too many other ways you can express the present value of a company other than what it sells for.