At the moment, there are relatively few exchanges where you can trade BCH, and the only BCH on those exchanges is that which was distributed by the exchange itself during the split - as there is no way to move BCH for now.
On the other way, people who want to buy BCH can move their money to those few exchanges, and inflate the bubble, hoping in the greater fool. Ergo, the $700 prices.
On the other way, people who want to buy BCH can move their money to those few exchanges, and inflate the bubble, hoping in the greater fool. Ergo, the $700 prices.