You cannot just have a c# variable in the .cshtml file and get it using javascript. Using the c# variable you get in the model, create a hidden field. Then use jquery (or javascript) to update the value based on the choice made in the select list.
I have created a sample that demonstrates this. Note that this is just an implementation using html & jquery. You will have to use this as a base for the mvc 3 implementation. Mainly, this is just to help you get the idea and not the full implementation itself.
http://jsfiddle.net/62qqk/[
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In case you have more questions post it here, one of us will help!
Hope this helps!