I generally advise against business models that are not sustainable. I use freshbooks but if the max you could make it 5000 * $5 = 25K that is likely not enough

I generally advise against business models that are not sustainable. I use freshbooks but if the max you could make it 5000 * $5 = 25K that is likely not enough to justify the cost of keeping an app current over a short app life of 3-5 years. If the app saves me an hour a month (or provides a similar time/value savings) I would consider paying more the $5. The $15 app is cheap for a niche app but I wasn't convinced I needed it.