I provide flat rate monthly support options for custom software I build. It includes:
- Bug fixes
- Email support
The amount is different for each customer, because some applications are more difficult to support than others. Also the pricing depends on how quickly I guarantee a response - some are satisfied with 2 business day response, while others are looking for a response within 24 hours, any day of the week.
Things you'll want to address in your agreement:
- Response time. (Make it clear that "response" isn't necessarily the same as "fixed" - you can't guarantee that things will always be fixed within a certain amount of time, as some problems may be outside your control.)
- What is covered. Bug fixes? Answering questions about how something works? Small cosmetic changes? And make it clear what are acceptable contact methods. I don't want my phone ringing at 3am, so contact is supposed to be initiated with me via email. (I might speak with them later on the phone, but I'm not taking calls 24/7.)
- How the contract ends. Mine are typically set up to require 90 days notice from either party, you may want to do something different.
Also, be cognizant of how much time this will really take. I set aside 25% of my time every month in case there are support requests, and I schedule other project work assuming only 75% availability. If support is needed for a client, I want to make sure I can still meet my regular project obligations. And I am careful not to take on more support clients than I can handle. Right now, I would not want to have any more than 4 support clients at any given time. Again - your circumstances may mean different limitations but consider what those might be.