Take a look at your 'retail installment contract' or your buyer's order. One or the other should show the actual sales price they charged you (usually line 1 in the itemization section). I'd think your discount would be the difference between that and the sticker price.
I do not, for one, think that the problem was that the band was down. I think that the problem may have been, that there was a Stonehenge monument on the stage that was in danger of being crushed by a dwarf.