Some compaines charge a percentage of the cost of the original product, say 15%, so if they £1000 for the app then the support would be £150 pa. That is of course what level of support you are offering.
You should offer free support for the first 30 days for firefighting then after that you need to discuss with the client what type of support they require, On-site , Off-site, Remote connect, telephone, email, etc.
Then you need to look at whether you are supporting the app only or the data as well. What disaster recovery options you have in place. What would constitute a call out and the turn around time. It all adds up at the end to an overall support cost.
You also need to build in any PCR's (Product Customisation Requests). Changes to the system that were not mentioned at the time of sign off.
David