Custom calculation with drop down and totals

So this may be a bit confusing but here is what I have, I made a custom form where the 6th column is the price of the line item (this price varies) and column 1 is where they select what group that line item is under to classify it. Let's say the groups are A, B, and C. I have a seperate box that I would like to take all line items with Group B selected in column 1 and add their column 6 ammounts together for a total. How would I do this? I been trying different calulations but have not been sucessful.