Sum Excel Formula

Hi guys,

Noob to the forum, so be nice please

I've got a problem with a spreadsheet, well, more specifically, with the results of a formula.

There is nothing wrong with the formula, it's a simple multiplication job, for example: =sum (c4*14.39)

The cell that i input the figure into, in this instance it's c4, is directly above the cell that displays the formula, i know there is no relevence to me telling you this, but that's by the by.

The problem i have is that once the figure i enter into c4 is above 5, i then need the multiplication formula to change, so lets say for any figure up to five, the multiplication figure is 14.39 and then anything above five is 18.00.

So, if the value entered is 8, the calculation would need to be (5*14.39)+(3*18.00) if you follow...

Any help or advice would be greatly, greatly appreciated.

Craig.