Compare data files

I have attached two zip files. They are csv files that i am trying to compare values and adding data from one to another. I need to compare the value of one part number to another, and if they are equal, then i need to add the -5% cost column to the sell price column of the other csv file. I have a cost file and a data file that i need to merge; merge the -5% cost column to its corresponding sell price column on the data file, mapping the part number associated with it..