As a general rule of thumb, consultants need to charge around twice as much per hour as the same work pays in a salaried position. So, if you think $15/hour is

As a general rule of thumb, consultants need to charge around twice as much per hour as the same work pays in a salaried position. So, if you think $15/hour is a reasonable rate for this "job," you should charge $30/hour. For a consulting gig, anything below $30/hour is basically slave wages anyway. Given that it is an OSS, it might be reasonable to charge anywhere from $50/hour to $200/hour. Since you have been doing this for free for some time, you should feel free to charge what you want. Anything is gravy at this point. You can keep it low as a favor to small businesses. Or you can jack it up to try to get people out of your hair. ;-)