The simplest version I could think of.

declare @t table (Qty decimal(12,3),CF1 Decimal(12,3),CF2 Decimal(12,3),Flag TinyInt)

insert @t select 10,2,6,0

insert @t select 10,2,6,1

SELECT Qty* POWER(CF2/CF1, Flag)

FROM @t

EDIT: Unless someone proves me wrong, it cannot be made without functions or a CASE statement. At least not in SQL Server (other languages don't need a POWER function).