Ask HN: Pricing for small business app?

My wife developed an application that her original customer uses to run his business. There's now another potential customer who wants to buy the package to run his very similar business. The problem is that we don't know how to approach pricing since the original was built as a custom consulting job. The software runs on a server and on laptops connecting back to the server. We're trying to decide if the best model is a one-time fee, a monthly cost per-laptop, etc. Is a one-time fee plus monthly maintenance a "normal" way of selling small business software? There hasn't been a ton of required support for the original customer but there's always new features wanted, bugs that pop up, etc. so we don't want to end up getting money up front and just losing money supporting the thing. Any ideas or past experience would be greatly appreciated. Thanks.

There's no problems with the original customer of the software in regards to selling it, in case that's a concern.