Very True, all that brackie says.
I think that you should probably add about 25c/l onto the cost of the oil for methanol cost. (That's at current methanol prices).
There are ways and means of purifying the used oil before esterification. 25c/l fuel has got to be better than 90c/l. This taken into account, i will have to be very careful with using used oil, and only use good quality used oil (if there is such a thing).
The main worry is water. Other muck simply drops out in the glycerine layer. If the biodiesel is carefully washed and dried before use, there should be no problems.
I don't intend to use virgin oil at all- partly because there is little gain in reducing fuel costs by 60c/l, when you add your (precious) time, electricity, and fuss, not to mention the risk involved.