Yup, it has a cost, but there's perhaps a one or two orders of magnitude cost difference between hosting instant messaging + calls with something like XMPP, and hosting mastodon/Lemmy/Kbin (or why I do the former but not the later, and why I'm ok to pay for the service, esp. considering that my instance's business model isn't, unlike Reddit, to re-sell influence and data).
Signal isn't a federated protocol, so even if they were incentivized to release all the server bits and pieces, it would not help. You could run your own, but wouldn't be able to reach-out to your friends running theirs.