If you have an old PC or laptop and are willing to take time figuring things out, a basic Frigate installation is free. I run it with Home Assistant and it provides completely customizable notifications to my cell phone with no cloud storage or servers required outside of some kind of SMS or push notification account.
Lots of cameras are compatible, but for a truly cheap option, TPLink cameras ($20) can be mounted inside your windows (with a very minor modification) as long as you have dim exterior lighting at night. I have 3 of these installed and they work fine the vast majority of the time.