Ethanol production: cost structure and margin per gallon

In a recent article, The profitability of ethanol production, A. Hoffmann tries to put clearly the cost structure of ethanol production per gallon. He states a gallon at $1.99 (ethanol price minus tax break per gallon of ethanol). But he integrates the cost of corn transport which makes sense I think.
A reader strongly reacts to its conclusion of a 30% contribution margin in the comments.

Unsatisfied with both their conclusions I try to recalculate this structure.
I prefer to keep the tax benefit because I don't think the current strong political support for ethanol is going to wane in a near future. Here below my results:

A margin between 10 and 15% seems more likely to my view point.

Sources:
SeekingAlpha- The profitability of ethanol production by Alexander Hoffmann (April 2007)
Determining the Cost of Producing Ethanol from Corn Starch and Lignocellulosic Feedstocks by the National Renewable Energy Laboratory (October 2000)
Fuel Ethanol: A Technological Evolution, page 8 by Novozymes and BBI International (June 2005)