Don't expect to run around to the local McDonalds and pick up the old
french fry oil and run it in your car.  Issues such as pickup, storage,
filtration, using the right kind of oils & fats,  and possibly having
blend with diesel will all add to the cost of converting "free french
fry oil" to a consistent form that will flow through the small orifices
of a diesel injection system.  Some restaurant fats begin to congeal at
room temperature.  To use those fats you would have to devise some sort
of heating system.

Quoting from the trade organization:  "Biodiesel is made from vegetable
oils and animal fats and most of the cost comes from the market prices
of these materials. In today's market, vegetable oils and animal fats
sell at higher prices than crude petroleum oil protecting our access to
foreign oils."  While they are in the business of promoting commercial
sellers of biodiesel, they make the point that biodiesel, like any
other fuel has it's own set of costs.

Here's some more information from that trade group on using biodiesel
fuels on passenger cars.

http://www.biodiesel.org/markets/pas/

Question for those with more knowledge of current fuel standards than I-
does biodiesel sold at pumps meet the same standards and regulations
(federal and state) as petroleum diesel, or is biodiesel allowed
variances of some of them?