Budget Blinds

He submitted his bill to me for all four and when I reused to pay the bill in full he said "oh your right the fourth blind will be considerably less." I in turn agreed to pay the bill less $100 and would pay any remaining when the fourth blind was installed. Now it is July 2013 and I am still awaiting the last blind I call again, advise him I am expecting company and would like this completed before they arrive. That doesn't happen, So late July he comes and installs the last blind, hands me a bill for $89.97 which is $10.03 less than the original bill. When I ask what happened to this blind being considerably less I am told that is how it figured. Now of a 53"X60" blind cost $200 would you think a 21"x28" blind would be less than $10.03 different?