The alarm control module can be programmed for up to 4 keys. The module is programmed with the transponder codes (the small 3mm x 2mm plastic coated chip which slides into the key head) and the radio remote codes. If a key is lost, the codes can be deleted from the module, thereby disabling the key from starting the car.
The two options you have with genuine keys are a full remote key or a 'service' key with no remote. As has been said, a remote key should cost about £150-200, and I seem to remember a service key cost me about £75 including programming. Porsche can cut the key from your VIN number. A good independent with a PST2 should also be able to supply and program a new key which will make it a bit cheaper. Porsche often want to see a copy of your V5C and driving licence as proof of ownership before they will supply the key.
Key cutters like Timpsons do it the other way around. They read out the code from your existing transponder and program it into a new one. I don't think they can do the same with the radio remote though.