Making your own biodiesel in a small batch process you'd be spending $5/gallon, if you remember to pay yourself or someone else for the time spent. If you don't care about the time spent, then the hard cost will likely come in at $1.25 / gallon.
Your setup costs will be in the $500 neighborhood for a good reactor.
Truthfully, you questions are much better suited for the Journey To Forever biofuels mailing list:
http://journeytoforever.org/biofuel.htmlFurthermore, depending on where you live, you could just convert the vehicle to run on Straight Vegetable Oil. See:
http://www.biofuels.caThe kit cost ranges from $300 for the most simple kit that will run just fine in a moderate climate to $1500 for a bombproof kit that will run in very cold climates.
Yet again, see Journey To Forver for answers to everything you could possibly want to know about biofuels.