The problem with the information you ask for is it is so variable.
Difficulty/price of conversion: depends on how much you can/want to do yourself and how good you are at the mechanical skills required
price per gallon: depends on how much you are paying for supplies to make the biodiesel, filter the wvo, how much you pay for the conversion, etc.
mileage difference: depends on the vehicle and how accurately you measure your fuel consumption, ranges from no difference to maybe a mpg or two less on wvo or biodiesel
availability:
www.biodiesel.org for a somewhat (I think) comprehensive list of retail biodiesel locations
practicality: see difficulty/price of conversion
etc:
www.biodiesel.infopop.cc