I put my rates in with a stack of decimal points, because accuracy matters!
But when I manually calculated the outputted costs associated with each transaction, I seemed to have some rounding errors.
So what was the issue?
The schema in MySQL was set to DECIMAL(10,4) which gives us 10 digits after the decimal point and 4 digits after.
A quick alter table and a reimport of the rates and I was on my way!
"alter table tp_rates modify column rate DECIMAL(10,10);"
Lesson learned and hopefully of use to any other CGrateS users who may be using MySQL as a StoreDB.