The cost depends on how you go about it.

If you want to run BioDiesel the start up cost is relatively low, $200 to $300 will get you going, but there is an ongoing production cost, about $1 per gal.

If you chose to run straight WVO the start up cost is high, as much as $3000 to convert a truck, and the ongoing cost is very low a little electricity and some filter elements from time to time.

You will find those that advocate blends, mixtures of WVO and various other liquid hydrocarbons. If you chose this method realize it is an absolute crap shoot you are gambling the life of your engine to save a few bucks. I for one will not take that gamble. They will tell you about friends they know that have run a mix of WVO and whatever for eons and have had no problem. That is anecdotal evidence not good scientific evidence and scienctific facts do not support their theories. I have a 4 year post grad degree with a minor in biochemistry. I will not run blends in my truck.