I'm doing something similar on a limited basis.
The way I've been doing it, is by product category. So I'm able to use dataport, export a specific category or sub-category. Then I export that to excel and with a simple cut and paste of the list price and cust price columns I'm able to do a quick calculation. Re-save the file, re-import to dataport and upload to

. It only takes about 15 minutes, at the most and probably closer to 5 to do.
I just re-do this at the end of the month and re-set the prices back to the original. If you want more specifics feel free to pm me.