Don't sell it.

If you have income coming in with little work, and it's growing, WHY would you ever want to sell it?

If you do have a good reason to sell, figure out you minimum (say, 250,000$), and ask for triple that.

Also remember you'll be paying lots of taxes on whatever you get.

So again, if it's really that easy to run, and it makes that money, just keep it. You can retire then, not even 1,000,000$ can compete with that.