You should check this related questionthis related question.
In summary:
A "lost bitcoin" is any unspent BTC balance in an account for which the private key has been lost. Without the private key, that balance cannot be spent again. If you lose your wallet file, you are effectively losing all the private keys for the accounts (addresses) created by that wallet. It doesn't mean that the balance cannot change, since the "lost" account can actually receive new funds (if funds are sent to its public address), but without the private key those funds will become equally inaccessible.
It's impossible for the network to know when a private key was lost. A balance can remain unspent for a very long time, but it could be by the owner's choice. So there is really no way to tell. No one can flag a wallet/account as lost, even the owner. This is because without the private key you cannot prove to be the owner anyway.
The Bitcoin price is simply a result of current demand/supply for it. You could say that the more lost coins, the less supply there is, so the prices will naturally rise, but that is simply a by-product of normal trading.