my old landy has a cav-lucas pump and ive ran veg oil and diesel mix of 50/50 during the summer for a few years now, its been to greece and back twice on this mix.
apparently what happens is the diesel lubricates the pump and veg oil isnt such a good lubricant so the seals become brittle or wear out, but i am no expert on this.
one thing to watch out for is premature injector failure and fuel filters clogging up.
always run a tank of straight diesel through it once in a while.
veg oil from tescos is 2.75 for 3 litres so you wont become a millionaire over night buy doing a 50/50 mix.
i do it because it makes my engine run a bit smoother , gives it a bit more power and a bit more mpg , but it is an old land rover so its quite a noticeable difference for me but maybe not really worth it on a regular vehicle.
also , dont do it during the winter , the veg oil has a lower viscosity when its cold and doesn't work as good as it does when its warmer.
did you know that the first diesel engine ever invented apparently ran on peanut oil?
you could try and grow some coconuts.
http://news.bbc.co.uk/1/hi/6634221.stm?lsm