What makes up the ATProto is more than one type of server. Compared to ActivityPub where you just run a single Mastodon instance and you're set.

If the thing you want to own is your identity and data, you can fairly easy run a PDS yourself. If the thing you care about is different moderation decisions, you can run your own labeller (moderation system) or subscribe to another someone else has made.

There's not one server instance type, there are different components to it

To 'own your data' you just need a PDS, which should be cheaper to run than an ActivityPub server

To build an app you need some of the more expensive parts