First, you need to decide how to defined the price of a coin, should it be:
The price of the most recent trade?
The average price of the most recent n trades?
A volume-weighted price of the most recent X units traded?
The midpoint between the current bid/ask?
You can get the data you need from Horizon:
Get the most recent n trades for a coin: example
Similar to bitcoin miners, a validator processes new ledgers (blocks) whereas a watcher just keeps track of what's happening in the network.
However to participate in consensus you'll also need to be trusted by other validators, so as long as nobody includes you in their quorum set, there is basically no difference between a watcher and a basic validator ...