£299.99 from everyone's favourite tax-dodging online megacorp. It's the basic model with 720p cameras, motion detection and night vision. There are improved models (pro and pro 2) which feature 1080p, audio (microphone and speaker), siren, rechargeable cameras and off-line storage. Pushes the price up a fair amount, depends on your needs. For interior use, 720p is adequate quality, you'll easily discern faces. I'm not convinced about sirens etc or the value of hearing someone burgle my house. I suppose you could creep them out with a softly spoken 'I can see you, yes you...' though I suspect they'd then steal the camera too.
For outdoor use and seeing someone at the end of the drive/garden, I'd be tempted by 1080p. Note for the non-rechargeable cameras, batteries are around £22 a pop (leastways for the official version, they'd the half-size AAish lithium ones). There are rechargeable and other options at a price. You can add cameras (so you can mix the basic and more advanced cameras) to the base station (there's a limit depending on base station).
Software is simple. You can arm and disarm online, set up geofencing, scheduling etc. Custom modes mean you can set for instance, the downstairs cameras to arm while you sleep and have everything come on when you leave the house etc, and there's a live view. Like I say, I'm not tested to see if the cat sets them off, our old place had a PIR system that was always triggered by the cats no matter how much we fiddled. In this case, if they do trigger, it'll just mean we have lots of footage of the cats wandering the house and a large battery bill, but hopefully we can tweak the settings to cat-proof. It claims to work with Alexa, IFthingy and whatever home control systems. There are online storage subscription packages beyond the basic free one-week service.
Tbh, I might have bought the pro version if I'd known how simple and effective it was, but the basic system is quite acceptable. The alarm at our old place, for comparison, cost about £5k and about £20/month for the basic call monitoring package, and we had to turn it off because it never completely avoid cat-based false alarms, so we could only use while on holiday as the cats were incarcerated.