Quoted from http://karting.4cycle.com/showthread.php?t=167398:
Q
How expensive is it to have a rod built to a custom length? A
That depends on the length, and we have to make a minimum of 3 rods. A bore diameter change at either end is $750 (for each end) unless it matches up with a bore of one of our existing rods so we can borrow existing tooling. A length change could be as much as $2000, unless it is only a slight center to center change that will fit within an existing rod profile. The same profile allows the use of existing jaws.
So, for the first 3 special rods, $600-$3500 (for all 3) depending on those factors. Additional rods made in the same run will be about $100 each depending on how many you get. There are other factors that could make it more, or less expensive, such as material cost and availability.