Miners have more coins then everyone else so they are the one who define the price (they won't sell if it's not profitable for them) , beside ... logically Bitcoin price is based on Supply and demand , so It's impossible for the supply to get halved and the demand stays the same or rise and nothing at all happens to the price ...
The amount of coins being mined is only a small part of the daily volume of coins traded. It is significant but it's a long way from being a definitive part of the supply.