Individual devices can range from dirt cheap to fairly expensive, but it all adds up over time.

Getting things initially set up is usually easy. Getting things automated correctly to your liking is a rabbit hole that is as deep as you want to dive into it.

What's really nice about HA is being able to do whatever the fuck you want whenever you want. You can take it in bites buying devices in waves and tweaking automations in waves.

That's how I've done it over the last 4 years or so.