if, isnumber and and formula in one cell

HI

I would like help with a formulae please using if , and and isnumber. On my two sheet I have a reference number, a cost centre and a cost, but not all reference numbers carry a cost centre so some cells are blank. I would normally use a lookup, but don't know how to add in the element to look for the cost centres, so that they report seperatly on each individual cc per reference?