I can make you biodiesel using waste oil for under $1/gallon using recycled (free) oil. There have been some recent advances which should push the costs down even farther(new catalysts for the transesterification process, etc) by eliminating the really expensive/hazardous ingredients like methanol.
How well does everything scale up? Beats me. We use 55 gallon HDPE drums (which used to hold udder wash) that we got for free.... in fact, most of our junk was free. I think I paid $30 for a triple beam balance off ebay, but that was about it... if you're used to a "cheap" plant, designing a new plant using large vats and holding tanks that were NOT free, I can easily see how some naieve hippy could underestimate the costs of a real plant.
So why is the finished product on par with regular diesel in terms of cost?
1. Why should it cost less? People are willing to pay $3 for dino, why not $3 for veggie?
2. The taxman favors using fresh virgin oil, not used (free, or low cost) waste oil
3. There are crops that can deliver more oil/acre than corn or canola, but they just aren't grown. The tax code favors certain crops over others.
There are probably other reasons as well, but there ya go. A coworker and I had a whole biz plan worked up for the biodiesel biz, but once we started seeing what some waste oil looked/smelled like........ugh......sick....... it's like dog vomit......so when you can spend you weekend turning dog vomit into diesel AND trying to figure out how fuel taxes work, or playing video games instead.... guess which one wins.
Maybe someday.... someone recently asked us to make some for them.... even offered to pay for all our equipment. Nifty. Too bad it was all free