Actually YOU need to define what level of support you are happy to provide within the advertised price - add an SLA and see if it works for you. You can also sell "support service" subscriptions with different levels if you want. It depends on the product.

For example, you know that your "Points with Levels" or "Classified Ads" modules generally need more support and potentially custom setups. You could be selling them for, say $3/month with 1-hour support SLA, or see more as "Ads+" with 5-hour SLA for $5/month... That's just a rough idea. It's really more about what would motivate your and where you meet on that with the client. Competition is there to keep in balanced and fair.