I don't know that either is more expensive to produce than the other, as each have their own issues.
A FFP reticle needs to be smaller due to its location in the optic, theoretically making the reticle itself more difficult to produce.
According to S&B, it's more difficult to make a SFP scope not shift POI during power change. In a FFP scope, the target image and the reticle are on the same focal plane and are magnified simultaneously. So, even if the center of the reticle isn't exactly on the centerline of the image, it doesn't really matter as long as it doesn't move. In a SFP reticle, it does matter if it's not perfectly centered in the target image because as you increase magnification, the amount of deviation error increases proportionally, causing a POI shift. Also, a SFP reticle element is located inside the zoom tube and can therefore move slightly when the power ring is rotated if the design isn't sound and mating parts are not held to very tight tolerances.
I don't know how much or even if those considerations have an influence on manufacturing costs, though.
Edited by RifleDude - January/26/2011 at 12:08