Earlier today, I saw a post on Hacker News titled "I sold my side-project for $30k".
Besides the point that the sale price was $10k plus up to $20k dependent on the growth of the sold entity, some in the comments worried that the sale would result in users becoming unhappy with the way the service was managed by its new owners.
That's a valid concern, as most acquisitions change the direction of a product, sometimes making radical departures that bewilder the original set of users—users who are often the most invested in what used to be a smaller project.
I've been running Server Check.in for just under eight years. It has a little over 100 paid users, and 30 of those users signed up in the first year.
I don't spend much time on maintenance, and the service has a 30% margin due to very low hosting costs, but I mainly built it to be a radically different tool than all the Pingdoms and New Relics of the world.