I should explain incase the fornula is errornous, by your own admission you say you are not getting the desired result as each part of the formula is being added together. I have encpasulated the entire formula in bracets having removed the + addition functions you've included.
Obviously, it's easier to work directly on a work sheet than it is to "pluck" a formula out of the air on a forum.
I have taken a look at the email, disregarding all the additional information am I right in assuming that the figure you are trying to produce is the basic cost + the additional percentage cost based on the two cells with the dimensions.
They are currently set at 450 & 1100 which means using the above assumption cell I5 = 321.99
Crosses his fingers in the hope that he has understood the problem correctly, that being the case I would simplify the formula to use lookup to either add or subtract the relevant discount.