I don’t recommend buying anything electronic off of eBay. A lot of it is counterfeit, a lot of it is mislabeled, a lot of it is used being sold as new. And many manufacturers will not honor the warranty if you buy it off of eBay because they consider it used no matter what the seller said.
So yes, it’s going to be challenging to hit your price point unless you happen to find something on sale.
What you are looking for is a device that has “dry contacts“, right?
In that case, there’s an existing thread for those:
Among other devices discussed in that thread is the monoprice door sensor P/N 24259. It lists at about $25, so if you wait for a sale it might work for you. (I don’t recognize the model number you listed. But typically if a sensor keeps dropping off the network the problem is the strength of the mesh, not the device. But it could also just of been a bad device. Lots of people use the monoprice sensors, it doesn’t seem to have more complaints than other brands.)
Also remember that for many of these devices, Amazon will not be the least expensive. Instead, you can usually get the best deal by going to one of the specialty home automation retailers. And if you need more than four or five of the same model, contact their customer service and see if you can get a quantity discount. Even if it’s only 5%, every little bit helps.
