You pretty much can't get a lower price than these Xiaomi and it is not the lowest price I've seen for them. Zigbee, not Z-Wave. Some have great success, some do not. I have no contact sensors, but I do have their leak sensors and buttons. Both have stayed connected, work every time and are well made.
I do recall reading some bad reports about Monoprice sensors, but then you'll find lots of bad reports about Xiaomi sensors too. If there is a money back guarantee, what do you have to lose. That's a really low price for Z-Wave sensors as @mike.maxwell points out. Before you click "buy now", I would check into what others have said about the Monoprice devices here and on the ST forum too, since the groovy code is similar between the two platforms. There's lots to read about the Xiaomi, but I have no idea about what info is available for the Monoprice sensors.
Start here
Feedback on pairing, performance and repeaters is here
[Update] use IKEA Trådfri outlets for a compatible and inexpensive repeater to get a consistent connection with Xiaomi devices.
