If you are selling products and services that have a wide price range, you may have issues trying to calculate your average order value. In this case, you may do 2 things. First, you can group them up into a few price categories and then do the calculation above. Assuming you are going to send them into separate campaigns, you should see the ROI for each campaign instead.
The second method is to just do a rough calculation of the average order value. The point of the ROI calculator is to give users an estimation of what they should expect to see from the results of their marketing efforts.