Formula Creation: Using List Function + Subtraction Function

Hi All,
I need help with what I consider a complicated formula.

I have two different scenarios, that I need put into a formula for entry on an invoice:
There are two different commission minimum types ($200 & $50), correlating with two different business types (Permanent & Portable). I have created a workbook with 2 different spreadsheets. The first spreadsheet is the entry sheet with room for entry of gross sales and verified cash, outputting the net sales, and sales percentage based on attendance. These all pull into the printable invoice, with more calculated information.

My question is this:
Based on the business type (right now is a drop-down list), I need to create a formula subtracting the minimum ($200 or $50) from the commission amount, to give an accurate commission amount.

Every permanent business is a $200 minimum and every portable is a $50 minimum, however I need to correlate this with the drop-down list, so it changes if the business changes...

I hope someone followed this enough to help me!
Thank you!

Also, this says I'm using Office 2003, but I'm using Office 2007! Sorry...

Unfortunately, I can't as the file is protected and the business won't allow it.