Jomres 5

Jomres' strength is in it's powerful and flexible rate management system. Unlike some rate management engines, Jomres offers a variety of ways to configure rates, allowing you as the property manager to structure the rates to as closely as possible reflect your real world tariffs without custom coding your own solution.

That's not to say that Jomres cannot be edited, it can. The system is PHP based, and the entire system is open source so you can edit the source files if you so require. Additionally, the system's various minicomponents (modules, or plugins, if you will) are licensed GPL / MIT so if you want to use some of those as a base for making your own plugins and selling them on, you can.