Yes BMW rip you off for a key most manufacturers are the same when it comes to keys, been caught before with a Kia we had with just one key. Trouble with doing it yourself is finding a key cutter that will cut blanks supplied by you, they of course would prefer to sell you a complete key. The key supplied that way can be synced to the car easy enough but that will only operate the central locking the key needs programing with the EWS module in the car to enable it to start the car.
https://www.ebay.co.uk/itm/New-BMW-...113247?hash=item3fcc4684df:g:5ZcAAOSwj35dfnPU
These people on ebay will do one key for 59.99 but you have to send your working key and the EWS module from your car to get it done, 1 day turn around service. Its not difficult to remove the EWS module its under the steering wheel held in by two nuts, just disconnect the lead and your done, you can secure the car whikst the box is out by using the keyin the door lock.
When it comes back refit the EWS box and check the key turns on the ignition. To sync the key to the EWS for remote locking put the key in the ignition and turn it to ignition without starting the car, remove the key holding down the unlock button and press the lock button three times in quick succession the door locks will lock and unlock to indicate that the key is synced. Immediatly repeat the button presses sequence with your other key to sync that one. Youtube videos are available to walk you through the process.