Measuring the contrast of an image
One simple way for quantification of contract that I can think of is through use of image histogram. Following is my suggestion


Compute Histogram of the Image 
From the counts compute entropy


If you just want to try it out you can use the matlab inbuilt function http://www.mathworks.ch/ch/help/images/ref/entropy.html

You can use the entropy value of the histogram as a measure of contract.