I just sent my single client a monthly subscription link via stripe.
$875 a month for service and support on a custom built loyalty platform.
Make about $25K up front to build it, then followed up with maybe another $20K worth of various features over the last ~5 years
Starting this month $875 a month to keep supporting them and helping grow his vision (credit management for events...ie: get a nfc wrist band at a party as part of an all inclusive package, has $xxx.xx worth of credit, tap your wrist band to pay)
i'm also starting to try to bridge this out as a more generic platform and look for new clients.
$875 a month in the USA is a nice bit of extra cash. Server costs are coming in around $300 because I haven't optimized aws and there are some legacy systems I'm supporting(VPC costs are killing me) but it'll be nice to have some extra cash to pay off some debt and maybe buy some things to fuel new hobbies.