If the only problem with the central locking is the key (have you tried re-initialising the key?) then, I would probably just buy a new key from BMW, and then keep your old one as a manual/spare if its broken.
If cutting the key in half, you can access, change, and reseal the original key, as long as the battery hasn't leaked and damaged the PCB.
I believe that in some (most?) diamond keys, the transponder is inside one of the chips on the PCB itself. The Chinese copies of the key don't have a transponder at all. Some suggest trying to extract the transponder from the valet key.. but if you've lost that, it wont help much
Even at main dealer prices, the Diamond keys are a fair bit cheaper than the newer bmw keyless opening keys. Last time I bought one for my E39, the dealers charged me £95.. Im sure they have gone up a bit since then, but it shouldn't be the end of the world, and BMW will cut the key to match your cars original locks, and all you need to do is initialise the remote control part.
Good luck whichever option you got for!