Advertising ROI in Appeal/Sources?

Hello all,

If you buy an ad in a newspaper/magazine/online which includes a promo code for the web, how do you track the revenue against the cost of the ad (ROI)?

Obviously the revenue is tracked by Orders using the Source code, but where is the best place to record the cost of the ad as an expense? Do you use the Acquisition or Delivery fields in the Source setup, or create a new Media type for every ad purchased?

I am sure this is a super common question that may have already been answered, but I couldn't find a thread about it.

Thanks all!

Nicholas