Add a hosted option with a monthly fee so people can try it out or even just have you host it.

I know that then its just like the others, but really its not like the others. The small differences are huge to a user.

A monthly fee will help stabilize the income.

Also, at $10/mo you'll make the same revenue every year as selling the whole thing. For comparison, basecamp is $29 or $79 per month.