You'll need some notion of how much time you'll be spending per month on this maintenance. Guess. Multiply that guess by what you think you're worth as an hourly wage, then double it.
Non-profits come in all sizes - some are charities with $0 budget subsisting on donations and volunteers. Others have enormous capitol assets and budgets the size of a small nation. If you're charging $300 per year, I assume this non-profit is of the former type, not the latter.
$300 per year barely covers the cost of maintaining your equipment, the electricity to run it, the internet connection, the chair you're sitting in, and the insurance that covers the edifice it's in.
If you undervalue your work, you bet others will too