The following summary is from "Pricing Corn Silage." (Ag Focus. August 2012. Pages 1 and 3.)
Summary

• Price analysis suggests that the price of corn silage depends on corn silage quantities, the price of alfalfa hay, the price received by farmers for milk, and the price of corn grain.
• Estimated corn silage price is sensitive to alfalfa hay price and corn grain price.
• Price estimates combined with understanding of relevant supply and demand factors from an individual farm business owner's perspective can aid decision making regarding corn silage price. Given current (May, June 2012) alfalfa hay and corn grain prices, price analysis suggests an estimated corn silage price of about $41 per ton.

Given recent market conditions, the first two points still hold, but the third would be revised to read

• Price estimates combined with understanding of relevant supply and demand factors from an individual farm business owner's perspective can aid decision making regarding corn silage price. Given current (January, February 2013) alfalfa hay and corn grain prices, price analysis suggests an estimated corn silage price of about $48 per ton.

Changing Market Conditions Since May, June 2012
For the May, June 2012 estimate

• alfalfa hay price was $180 per ton, rounded up from 175 reported by USDA/NASS (Agricultural Prices. Washington, DC. May 31, 2012.)
• the price of corn was $6.50 per bushel, rounded up from 6.18 reported by Western NY Energy ("Corn Bids." June 12, 2012.)

Since the May, June 2012 period, corn grain prices rose to around $8 per bushel and then declined to current levels of about $7.15 per bushel. Alfalfa hay prices have risen to about $250 per ton using the USDA/NASS source.
Corn Silage Price AnalysisEmpirical price analysis suggests that corn silage price is a function of corn silage quantities, alfalfa hay price, the price received by farmers for milk sold, and corn grain price. Ordinary least squares regression provided an estimate of corn silage price as a linear function of the above variables. Even though the analysis is somewhat rough, elementary, readers of the August 2012 article note that the analysis and estimates generated should be helpful to farm business owners looking to price corn silage.
The New York State Agricultural Statistics Service is the source of market year average price and quantity data for the variables listed above for the period 1991 through 2010 (http://www.nass.usda.gov/Statistics_by_State/New_York/index.asp). 
Updated Corn Silage Price EstimatesCorn silage price estimates can be generated using the ordinary least squares regression results reported in August 2012, where estimated corn silage price is a function of alfalfa hay price and corn price, other factors (corn silage quantity and milk price) fixed at average levels for the period 1991 through 2010.

• estimated corn silage price ($/ton) = 10.621 + (0.079 x price of alfalfa hay ($/ton)) + (2.448 x price of corn ($/bushel)).

Suppose

• alfalfa hay price is approximately $250 per ton (USDA/NASS. Agricultural Prices. Washington, DC: National Agricultural Statistics Service. January 31, 2013.), and
• the price of corn is roughly $7.25 per bushel (Western NY Energy. "Corn Bids." February 11, 2013. Approximate value of those actually reported.)

Using the estimating equation and the above prices for alfalfa hay and corn grain yields an estimated corn silage price $48 per ton.
Estimated corn silage price is sensitive to alfalfa and corn grain prices. Suppose alfalfa hay price remains at $250 per ton, but corn grain price is $8 per bushel. Then, the estimated corn silage price is $50 per ton. Recent conversations with producers suggest alfalfa hay prices higher than $250. Suppose alfalfa hay price is $300 per ton, and the price of corn grain is $7.25 per bushel. Then, estimated corn silage price is $52 per ton.
Corn silage price estimates combined with understanding of relevant supply and demand factors from the individual farm business owner's perspective can aid decision making regarding corn silage price.
For more information please contact John Hanchar.
Thanks to Christian Yunker, CY Farms, LLC/Batavia Turf, for providing valuable comments on earlier versions of this work.