Biodiesel from algae is still awhile away, and swithgrass is used for 2nd generation ethanol. The tax credit alone does not insure a biodiesel operation will survive. 2009 was a terrible year for biodeisel with the falling petrol prices. An operation must also have an efficient conversion process and a favorable feedstock market. However, high petrol prices, such as we see now, combined with government incentives make it almost impossible to fail. I believe XPGH is using waste vegetable oil, so they will only be limited to the WVO supply.
To answer your question, natural gas is very affordable but probably not going to affect XPGH's fuel. Waste motor oil is much cheaper than biodeisel, but supply is just not there outside of heavily urbanized areas. XPGH is competing with diesel fuel in deisel-fired industrial boilers. At least that's my understanding from what information the company has released.
I always thought their glycoal product was the golden goose for this company, but I haven't heard anything from the company regarding that in awhile. Commercialization of that product would've allowed them to compete with coal.