Try this:
The markup value:
Select C2
FormatCellsNumber
Category: Custom
Type: [<=1.5]0.00%;[1.5]$0.00;General
Click [OK]

Then the markup amount:
D2: =IF(C3<=1.5,B3*C3,C3)

The Total Price
E2: =+B3+D3

By doing the above, if a user enters a value less than or equal to 1.5 as a
markup...it is assumed to be a percent. Amounts above 1.5 are assumed to be
dollars.

Does that work for you?

Regards,
Ron