The TPMS sensor unit is just like a tyre valve but at the bottom, the bit inside the tyre, is the encapsulated battery, pressure sensor and radio.
They are fitted by removing the tyre, fitting the sensor through the valve hole from the inside and then refitting the tyre. Then inflating the tyre.
So unless you have tyre removal equipment, it is not really a home job.
There seems to be development work going on all the time on these things, along the lines of battery-less, or at least self recharging sensors, and universal sensors which will probably be cheaper than bespoke BMW ones.
Bit of a pain really, thanks to the EU for making them compulsory over here!