Its a good Question, I think it is not discussed so far at any where.

EMV 
Is a decision method and 
Questioner is linking it to contingency cost 
If we look 
Reserve Analysis in Estimate Cost 
It clears how contingency budget arrives 
Its
Either
. An assumed lump sump %age of each activity cost based on SMEs.
Or
. Estimated by using quantitative analysis methods 

Now question is what is that quantitative analysis methods - Is it EMV - decision tree method.
Then which value will you consider - Impact or score value ( i x p)
The questioner has stopped here perhaps with these 2 options. 
My answer is - it is only decision method to choose the right options among many available with their pros and cons . ( Pmbok note no 1 figure 11.15) and not a contingency reserve estimation.

Then how will arrive that quantitative amount of contingent.
There is an illustration in figure 11.16.
This illustration says 
Suppose estimated cost of an activity or a work package is 41M, and then organization likely hood of success is 100% or 75% and then its amount as per simulation is 60M or 50M respectively.
Then contingency will be 
60-41)/41 = 46% or 50-41)/41 = 22%.
Hope it is clear.

for more clarification read PMBOK fig 11.16