'The key' is, in essence, in 3 parts.
-the metal blade, operates the mechanical levers in the locks
-the immobiliser chip, housed inside 'the key'
-the 2 or 3 buttoned 'plip', which from MFL-onwards is integral in 'the key'
Depending what you actually
want the spare key for... you can get a simple, bog-standard blank cut at any decent keycutter for maybe around £20. this should get you one with the chip in it specifically for deactivating the immobiliser, so you can start the car. No remote function. Using this key you'd have to actually unlock the car by turning the key in the door barrel. The alarm will sound until you turn the key in the ignition, as the car thinks it's had the door forced. That's only what you'd have to do in the event the battery dies in your remote plip anyway, it's 'no biggy'
If you want a spare key in the event you lose your own and desperately need to get in the car until such time as you locate said original key, then this is perfectly sufficient - after all you may never, ever need it.
If you want this spare key for a partner/whatever to use of a regular basis, then you'd probably want to go up a notch. Get the blank key blade cut, that's the one with the c-shaped plastic section attached, plus chip; then contact an OOFer endowed with a Tech II who will be able to programme you a working 'plip' (you source your own plip/remote part of the key, often literally a matter of a couple of quid) all-in this will be significantly cheaper than the £115 on offer.
Third alternative depends of if you fancy getting your hands dirty...you can swap over the keys, chip and plip, plus the bits on the car that receive the digital info, taken off a breaker on here, for most likely around half the cost of the locksmith's charge for the one key.
Depends how far you want to go, each direction has its advantages and disadvantages.
