I tried mucking this around as it has promise but really cant get it to
work.

=IF(ISERROR(C10),"",$A10*D$3,($A10-((D10/2)*$A10))*D$3)

to

=IF(ISERROR(C10),"",(if(c10<"",$A10*D$3,($A10-((D10/2)*$A10))*D$3)))

Which worked :) Thanks so much for the headstart!