Currently it's making about $5,000 / month.
It's been a very interesting process dealing with building the app, marketing it, and dealing with support. The app is $19/mo for regular Shopify stores, and we have about 700+ total customers (only some are paying). We get A LOT of support requests for misc things and have been trying to build up a knowledge base of articles and help guides in our ZenDesk.
We have recently been running into an issue with file hosting storage fees (we allow people to upload files, no additional charge). We have a couple of Shopify stores that are costing us about $70/mo each in file hosting fees (we don't charge them extra).
I absolutely LOVE building apps for 3rd party providers. We don't really market the app, it's found organically in Shopify's app store. The first 8 months were filled with lots of little bugs needing fixing, but that has since slowed down as I've fixed things. At this point, it's a pretty stable application that doesn't require much maintenance, mostly just user support.