Cheap and reliable don’t always go together. 
Right now there are three popular solutions for those in the U.K. for whom cost is a primary factor:
1. Very inexpensive, somewhat wonky.
Xiaomi sensors are very inexpensive, often £7 to £9 from gearbest.com , but most of the models are not certified to the zigbee home automation standard, so they are somewhat “idiosyncratic.” Pairing can be very difficult. After you do get them paired, you may find that they don’t work well with all your repeaters, which can cause messages to get lost and status to fail to update, or they may drop off the network all together. And at least once a platform update from SmartThings caused all of them to fail at once. So for those who are willing to run the risk, these can save a great deal of money, but I’m not sure we can call them “reliable.” It may also depend on your specific network layout.
2. A little more expensive, a little more more reliable.
Orvibo is another Chinese brand, but they have gone to the trouble to get their devices certified for “zigbee home automation,” which is good. However, they are very cheaply made, which means whenever they have trouble of any kind they can either fall off the network or start running through batteries very quickly.
Also, for a while they had problems if there were Osram/Sylvania zigbee bulbs nearby, although that problem has been improved by recent firmware updates to the bulbs.
Anyway, I believe @a4refillpad has used both the
Orvibo and the Xiaomi and may be able to say more about current thinking on these.
From an engineering standpoint, it looks to me like the Xiaomi are better made but are not following the standard protocol, which introduces problems. The Orvibo have gone to the trouble of getting certified to the standard protocol, but are just inexpensive devices made inexpensively, which reduces their reliability.
3. Moderately priced zwave devices, high reliability.
An alternative is to go to one of the moderately priced Z wave brands. These will all cost about triple the price of the Xiaomi, but will pair easily and perform more reliably. Typical costs are between £25 and £35, But if you shop around you may find them on sale from time to time.
Brands in this group include Neo Coolcam (zwave plus, which is nice as they will have better battery life and better range than Devices using the older Z wave classic chip), DLink’s zwave sensors, Everspring, Teldus and Vision. Philio sometimes priced in this range and sometimes 10 pounds higher.
https://www.amazon.co.uk/Neo-Coolcam-Wireless-Security-Garages/dp/B06XKQFQGN/
The inexpensive Z wave sensors will all tend to be somewhat bigger than the inexpensive zigbee sensors.
Make sure you get them on the EU zwave frequency (868.42) and not the US Z wave frequency (908.42) as they must match the frequency of your hub exactly.
There are lots of other zwave brands as well, it’s just that the others tend to be more expensive. The premium brands that everybody knows are Aeotec, Fibaro, and Sensative, but there are quite a few mid range brands, too.
Also note that sensors do vary in the features that they have. Some will include temperature reporting, many of the inexpensive ones will not.
So from an engineering standpoint, I think the least expensive reliable sensors are the lower-priced Z wave ones. But if you’re willing to risk less reliability, the cheap zigbee sensors are much less expensive.
One more thought: if the piece that you lost is the smaller piece, that’s just a magnet in a fancy case. You can use any magnet with the sensor and it will work just as well. Sometimes people will swap out for a stronger or weaker magnet in order to fit a particular use case. So that part is easily replaceable. 