Live market pricing usually has a cost associated with it. Stock and commodity exchanges charge a fee for their price and volume data, which brokers pass on to their clients.
You can go online to get free delayed quotes, which are useful for some analysis, but you can’t trade successfully in real time if you give the rest of the market a ten-minute head start.
On Nadex, the price quotes are live but rather than tracking only the most recently printed trade, Nadex uses a continuously updated average of the most recent prices. The calculation is the same one Nadex uses to calculate the expiration price (which is compared to the strike price), but it’s done on an ongoing basis during the life of the contract.