Prices on gearbest, aliexpress, banggood and … change from day to day, there are plenty of discounts and things like this. So you can not say, the old sensors are more expensive. Only wait for the next discount and they are cheaper.
I think I read somewere that the aqara door sensor has a light function or some additional feature, but I’m not sure.
I can confirm that the aqara temp/humidity sensors can measure the air pressure too in OH 2.1
please take your time to read the docs - if they’re unclear, tell us please!
besides: without the gateway the sensors are useless. not only for OH2, but they don’t have the opportuinity to report their status somewhere (normally, you add the Gateway to the MiHome App and then the sensors within the App).