Sorry for the delay. The monthly costs (which are actually closer ~$800-1000/month), are a small part. A bigger worry is if we take money from people, we really have some obligation to provide "reasonable" service. We've had relatively few system-administration incidents so far, but I'm concerned about something happening when we are on vacation or busy with other things.

Worst case scenario is Chrome changing how extensions work, which requires us to actually write code, or someone finding a serious security vulnerability.

As a conclusion: It really would take more like a total of $3000/month in fees to make it worth someone's time to deal with the paperwork, the administration, and to be willing to be on call. It seems unlikely we'll get there, but I'm investigating the possibility.