This is your baseline - BMW TPMS Tire Pressure Monitor Sensors BERU

If you want to get them cheaper, look for used sets (forums, ebay, CL, etc.), but keep in mind while the sensors may be the same, various BMW/MINI models used different valve stems (43 mm vs.49 mm), which you may need to get separately. For a used set I'd also recommend a rebuild kit just cause it's cheap.