Bit on the steep side at £180, you can buy a new blank off ebay that comes with the remote buttons and a blank transponder for little money but then have to find a tame key cutter who will cut your key for you, many won't cut your own keys. Then you have to get the transponder coded to the car so it communicates with the EWS system for unlocking the immobiliser to allow the key to start the car. Can be done using an Ak90+ key coding machine also on ebay if you fancy a go. I had no joy using the AK90 machine and ended up having a mobile locksmith come and code the key using his 3K machine that he wired to the eeprom chip inside the EWS module to code the key. Cost me £130 for his service in the end to get the key working.
Other option would be direct with BMW, check out what they want for a key, think it will be less but may only be a dumb key. Plenty of other BMW key replacement services out there on the Web but usually involves sending your working key and the EWS module to them if your up to the risk of loosing them in the Postal service!!
Blank key here