How BTC value is determined in USD from a trading pair? I know this is more of a general trading question but how is BTC or any base currency value determined in another currency from a trading pair?

An example of what I mean is take the BTC/NEO trading pair, how do you determine the BTC value in USD from that trading pair?
If one bitcoin is $1,000 (for the sake of easy math), and 1 Neo is worth 0.1 BTC, then you can determine the USD value of one Neo is $100. 

1000 * 0.1 = 100