You can look at ETH vs ETH classic to see the effect. Initial huge drop while things get reorganized, then after a long time, with the legacy chain worth 10% of the main chain, with unknown extra value lost due to the split.

This assumes enough hashpower where people can still transact on the legacy chain, and with some sort of replay protection to where exchanges can safely actually convert the funds to have two separate tradable tokens. Right now that doesn't exist, so exchanges won't touch it. This is a huge reason why Bitcoin Unlimited was a non-starter.