How is MEF calculated for Canadian VNC? According to the VNC, the MEF represents the highest feature in each quadrangle. It refers to 


AIM RAC 5.4 and CAR 602.15(2b) which basically says "lower than 500 AGL (CAR 602.14(2b)) ok, when for aerial application or aerial inspection, photography, ..."
AIM AIR 1.5 which is about "Pressure Altitude" which I guess is because the heights are ASL


Now, the sample quadrangle here shows a MEF of 2700'. Looking at that quadrangle, the highest obstacle I would say is the mountain in the very east, north of 50°35' which is 2290' high. I wonder where that difference comes from.

I think that both figures, 2700' MEF and 2290' are ASL. Therefore it can't be the difference between ASL and AGL.
It always seems to be ~400' more than the highest feature.
According to Aeronautical Information Circular 26/11 VFR Navigation Charts—Clarification of the Maximum Elevation Figure the 


  The MEF is calculated by taking the higher value of:
  
  
  the top elevation of the highest obstacle plus the vertical accuracy (variable) of the terrain
  source data; or
  the elevation of the highest terrain plus 328 feet plus vertical accuracy of the terrain source
  data.
  


So the 2700' could come from 2290' plus 328' rounded up to next hundred feet.