[davidmanescu]

Very true - neither the person who wants BTC (but has no opinions on BCH) nor the BCH buyer wants to take the risk that the other coin will drop before he/she can sell it. In other words the risk premium isn't zero.

However a rational (impartial?) actor should be willing to take that risk if the premium is big enough. That could mean buying pre-fork or selling pre-fork if there is an obvious opportunity. Which means in an efficient market the risk premium should be zero "on average". It's hard to reason about averages with one sample but that sample was distinctly not zero!