As I understand it, Bio-diesel currently only uses about 5% bio fuel with 95% conventional diesel. The tax advantage only applies to the bio component, so commercially available bio diesel is only about 1% cheaper than ordinary diesel.

This may change as with more experience the pecentage of bio fuel in the mix is likely to increase, and its production cost relative to ordinary diesel may drop if the oil price continues to increase, but at the moment its only real advantage is a (nice?) smell.

Rob