Arwen trades are priced from the centralized exchange's orderbook.

Currently, Arwen supports Request For Quote (RFQ) trading instrument. 

In an RFQ trade, the exchange commits to a price, called the quote, before Alice decides whether or not to place an order for the trade. (Request: “How many BCH can I buy for 2 BTC?” Quote: “You can buy 40 BCH, quote open for 1 second”) If Alice places the order before quote expires, Alice cannot back out of the trade and the exchange is expected to execute a trade (of exactly 2 BTC for 40 BTC).