Customer Analysis and Quote Tool

Hello. I am just learning DAX and have been using Excel for a while, and just starting to learn Power BI.

I am in the internet industry and have a table of my customers (lets call it 'Sales') that shows their current usage (in GB) and Charges by product type and region. I have created a calculated measure to get the current cost per GB, and I can drop that in a Visual and it will show the rate per GB by region, as well as total charges and usage by region. 

What I am trying to create is a dashboard that will show my customers potential savings if they commtted to a monthly volume of traffic. I have a table of discounted rates per region ('Rate Plans'), where you have a Plan ID, Region, Rate. I figure I could easily run some DAX to caclucate the current usage by region x the corresponding discounted rate in my table. However, there are times when I want to override the fixed rate in the table with a custom rate. 

It does not appear that I can just create a data entry field in Power BI with the rate I would like to offer and have that rate override the rates in the Rate Plan table. Seems like if I want to do that, I would need to create my tool in Excel.  My thought to do the same in Power BI is to create a DAX table or measure that applies a discount to the rate or adds to the rate in increments of $0.0001 and add this as a slicer or filter. It would have to run on each region idependently. So, one slicer or filter within a visual per region. 

Am I asking  too much of Power BI. Should I just try to do this in Excel?