The cost of using the Relay service has two factors; the average number of concurrent users in a given month and the amount of egress data. Both of these factors include a free tier and you will not be charged if you do not exceed these limits. For the most up-to-date information and exact pricing, please refer to the Unity Game Services Pricing page.
Concurrent Users (CCUs)
Once the service has been activated and configured for your project, Relay will frequently record the number of active users. At the end of each billing cycle, these numbers will be used to calculate your average user count for the cycle.
Egress data refers to the total amount of data leaving the Relay servers. For example, if you were connected to a game that had four players and had one player transmit 1MB of data, that data would be propagated to the three remaining players totaling 3MB in egress data. Note that the pricing for egress data may depend on the location of the Relay allocation.