So I recently started my relay using an idle raspberry I have.
It has been running for 32h but transfered about 8gb of data.
In this time the cumulated CPU usage was 3h, so it was bored for 29h. With the connection I have it would take less then 15mins to transfer 8gb of data so it’s obvious that I could contribute more with what I have.
Bear in mind that raspberry pis aren’t very powerful, and neither are home internet connections. You don’t want a large number of connections, as the more connections you have, the worse the experience will be for each of them.
Also note that most relays are VPSs running in data centres. Relays are picked based on their ping. A VPS in a data centre is normally going to beat a raspberry pi on a home connection.