I'll post this as a top level comment, because I think it is crucial: A few people are saying that it is expensive to run a relay but others have done for as low as $34/month [1]. So unless somebody presents other proof that it is expensive I would say that those posters are either wrong and trying to mislead us on purpose.
Edit: actually the article links to somebody doing it for $18/month.
I've also done a full network replica (all the data indexed in a postgres) on a raspberry pi (with an 8tb nvme attached via a hat). Its really not expensive to do . And if I wanted to drop data older than say 3 months, it would be even cheaper still.