“Priceless” financial contracts are contracts that don’t require an on-chain price feed to function, and minimize on-chain oracle usage to reduce the frequency and surface area for oracle attacks. They are designed with mechanisms to incentivize counterparties to properly collateralize their positions without requiring any on-chain price feed.
These mechanisms include a liquidation and dispute process that allows counterparties to be rewarded for identifying improperly collateralized positions. Unless a position is liquidated, it is assumed to be solvent (properly collateralized). Oracles are only used when a liquidation is disputed — which is designed to be rare.
Here are some additional resources to look at to better understand how the priceless synthetic token contract works: