The cost of the 968 motor can buy you an LS1 but there is alot more to an LSx (or any engine swap) than just buying the motor.
A 968 motor is direct fit.. no need to figure out power brakes or adapt the LS based power steering pump to the Porsche rack. Not to mention all the other parts you would need for the LS swap; oil pan, engine mounts, headers, bell housing adapter..etc.
Im not saying dont do an engine swap (see my other thread
), but look at the big picture.