Sidekiq (http://sidekiq.org/), a Rails job queueing system, works on related model: open core, paid subscription if you want to host a version with all the commercial bells-and-whistles plus get support. The author does well for himself, deservedly.

I'd consider shipping a bare bones version of the back-end under MIT, letting people spend their own two weeks getting one ready if they want to have it more useful in production, or give them the option of paying $500 per year for your back-end. That's an easy, easy call for a lot of companies to make.

Just to set expectations: just OSSing your thing is not going to cause the world to beat a path to your door. You're going to have to go bang down doors to get this adopted, whether it is 100% free, 100% paid, or some combination of the two. Unless you put in the marketing and "sales" work, you will probably build no community, receive no buzz, acquire no press mentions, and get no meaningful support from other developers. Be ready for this, because three months from throwing most projects on Github you can expect a deafening cacophony of absolutely nothing at all unless you find the people it solves a problem for and, with the best of intentions, shove it down their throats.