[CorneliusRox]

I'm not sure on the cost difference, but a 13mm pump has a lot of benefits over fuel volume. It'll move 17% more volume on the same cam profile as a 12mm pump, which means your injection event (timing) will be much more precise and you'll get more fuel into the blow (which adds more efficiency). You'll be pulling less timing because the fixed timing will be more accurate.

Say a 12mm pump at 3,000 RPMs takes 8deg of rotation to inject, where as a 13mm pump might only take 6.8deg, you can more fine tune your system.

Basically, you can keep very stock fueling with picking up power and responsiveness.