First, talk to you your potential customers (i.e. the current users of the open-source product) and find out answers to the following questions:
- How often do you find yourself needing technical support?
- How much do outages/problems impact your business?
- How quickly have you resolved your outages/problems?
- How quickly do you want your outages/problems be resolved?
- How complicated have your outages/problems been?
- Where did you get technical support?
- How good was the support received?
- How do you budget for technical support and outages?
With answers to these questions, you'll be able to craft your UVP and figure out the value (not the price) of your service to businesses.
Then you'll know enough to pick your pricing model from the typical ones: flat rate per incident, flat rate for the first X hours then hourly, only hourly, retainer (monthly/semi-annual/annual with/without incident limit).