The recurring license revenue is nice. The billable hours are a little difficult to value though since they are very dependent on you skill set. If you are doing something fairly unique, and the buyer can't be trained easily or successfully than it might be hard to honestly justify that revenue when negotiating a business value and price.
What are you marketing and advertising costs now? If you say "basically zero" then you might have a lot of untapped potential value.
Since you are not interested in running a business and hiring people, I'd suggest selling now. It'll take a few months to find a buyer and a few months to close a deal so might as well start now.
To paraphrase Warren Buffet "no one ever lost any money by taking a profit".
On the other hand, $2k per month of passive income while in grad school is a great way to live and study and work without a lot of added stress.
I wouldn't take less than $125k for it, cash (don't carry a note for the buyer). And offer the buyer 40-80 hours of training over the first 2 months.