If you truly want the lowest price, then I doubt you will be able to beat an ESP8266 based board(I like the Wemos D1 Mini) connected to a door sensor. That will probably run you like $7. And then a few bucks more for a microUSB power supply and cable, which most people have a box full of.
It definitely requires some DIY, but they are awesome little boards that can do a lot.
i use arduino Nano’s
is about the same price
the only problem i have all the time is to get power supply on the place where i have my sensors and that its Always a bit bigger then the sensors which are complete already.
Love the idea… good for a play and for the toolshed, I dont think it passes the wife stamp of approval with the aesthetics of my poor workman ship and lack of a shell all over the house though 
It also wont fly anywhere a baby can reach …
The Xiaomi actually look reasonably slick for $10-12 worth the extra few dollars (assuming we can get them natively intergrated into HASS)
Fingers crossed we get them going!
i make it all wife proof by making a nice box around it. 
i dont have a baby, but most if my stuff is at high places anyway, because it is easier for sensors like motion detectors.
but you are right, 10 to 12 bucks is a nice price and it saves a lot off trouble.
allthought there are still a lot of other advantages with using arduino.
i am implementing them for switches, but i also use temperaturesensors, lightsensors, motiondetectors, distance detectors, heat and gas detectors at the same places.
giving me a shitload off information around the house which i can use for my automation.
hmm. the motions sensors are USD 9, or AUD 13. and they are battery powered, and look neat. They are not perfect by any means, but these are the cheapest commercial solution i have seen, and should do what you need.
I am pretty confident that they will be integrated. So far they do work using @jon1012 python script. And it think @fooxy has it working in hers (His?) custom component. so i wouldnt worry about it not being implemented, but when.
And the WAF is extremely important. Nothing goes without WAF. Once you add a nice case and power supply, the DIY is more expensive (though much more flexible). And if you have a baby - if you’re like me, you wont have time to DIY…
what is the link to buy a hub/gateway XIAOMI? i trying to find, but no luck.
Gear best.com have them but I think the kit is sold out - I bought all the items individually
if i buy the wireless switch with yeelight… the home assistant knows if the yeelight is on or off?
if i buy the door/windows sensors, and the motion sensor,the home assistant knows if the door/window is on or off?
Door switch integration is hopefully coming soon…
My waiting on my yeelight when I get it I can confirm with you 
aliexpress has gateway kits that are reasonably priced. However it does take forever for shipping. I bought individual sensors from gearbest as well. paid a little extra for faster shipping and arrived in a week and it was during Christmas peak too
where do you live?
Try
Banggood (slow for Italy)
Gearbest (very fast with express option)
Fasttech (seems very good, testing now)
Aliexpress (I will never buy again from them, poor customer support)
Any feedback / experience gained since this topic started?
I don’t think the DIY solutions would be relevant let alone look acceptable to deploy all over the place - I tried a basic 433 sensor (the 6$ type by KERUI) along with a Rpi & RF sensors attached + some code I found online. I managed to read the signals but they seem to be weak and undependable…
SO… What is the most cost effective solution for “Door / Window Sensors”?
Yep I got the xiaomi sensors setup!
They need a hub but its only $30
@rave setup a great intergration check the thread linked above
Seems to work for automations for everyone I’m still setting things up
Range is around 10-30m I found but you can use multiple hubs and they suggest the mains smart plugs work as an extender
Working great for me thus far.
Here we go
I am late in joining the xiaomi party I guess… But folks, to my experience, its been more than awesome for me… I tried with multiple sensors(2 PIR Motion, Temp/Hum and Lux) approach before with NodeMCU then pi Zero. Both sort of worked, specially pi zero with python(paho mqtt) worked like charm for couple of hours but the reliability of the motion sensors actually put me off. After burning 2 NodeMCU and 3 PIR sensors, I quit with that route. The problem I encountered, a lot of misfiring with the PIR sensors which I was using for multi purpose e.g. If day time and we are home, the motion doesn’t do anything, if we are not home then it would trigger alert and if it dark outside and we are home, then it would turn the light on.
The conditional parts, homeassistant could reliably handled indeed but think of a number of false motion is detected every now and then!
Last two days, I am using the Xiaomi motion sensor and to my surprise, so far it hadnt trigger a single false alert!!! I am so excited that ordered a bunch of those sensors… Now only missing sensor is the lux sensor which can be readily integrated with my pi zeo or NodeMCU but that would require power source which I am trying to avoid. If anyone has any idea about such lux sensor (zigbee based) please give me a buzz.
Well, the gateway itself has a lux sensor. (Lux, lumen or something similar, I don’t think we have reached consensus of what it actually is.)
Thanks for the info. From the app menu and seller’s description, i sort of guessed that’s the case… But I didn’t find it explicitly anywhere in the app that is reporting the value. May be it is but I didn’t understand, can you suggest any method to pick that information as a value not approximation like dark or not_dark.
Thanks a lot man for re-directing me to the right direction. From the long discussion and numerous number of forks, it took a decent amount of time to figure out which version is the right fit for me Btw after using your recommended version it was up and running in no time… Great work indeed by @Danielhiversen . I checked his repo as well and was a bit lost to see his massive volume of works…
By the way, after using this version, my reported battery was instantly reduced to 49% from 51%. Do you know if it is just because of how it’s shown/truncated rather it actually reduced 2% due to polling ? I am not clear on this… May be I should wait few days to see how it goes…
Secondly the light sensor reading from the gateway is somewhat erratic sometimes… Specially from complete dark when I turn on the gateway light, it immediately drop to ~330 range from ~420 range. Other than that its pretty solid to report any increase. And as I read from the forum, the lowest is ~300 ? This one is actually quite different from the previous chip I used (BH1750) which can report very precise values in my case from ~4 to ~48000 Lux.
Thanks again…