DAX: Calculating different rates on user defined inputs

A few weeks ago a client asked me an interesting question. They had a table of data, and they wanted to pass in (in their words) a variable in order to change the calculated value. I'd played around with banding before (see my post here), but I hadn't been able to play with something quite as specific as the scenario they presented. To outline the issue, let's say we have a user created table that consists of the different pricing tiers for our customers, similar to the below: