I'm a bit late, but if it helps, I've had good results with the Sonoff and Tuya zigbee sensors. I prefer the Tuyas, just because they use AA instead of CR2032 batteries.
$50 will easily get you everything you need to start.

You might also want to think about getting a 433MHz radio for the system.
If you live in a built up area, you'd be surprised how many people have equipment transmitting on 433MHz already (free data!).
And a lot of generic wireless temperature sensor kits use it as a standard, making it a good value way to get weatherproof gear.
I got a generic fridge/freezer kit for about £10 that works perfectly with this. Again, $50 or so for a DVB-T2 receiver. Just be aware that it's a little more technically involved than pairing zigbee devices.

Bother me with DMs if you like, I've been fiddling with just about every protocol I can get my hands on for under £100!