If you do still buy the car, get the key reprogrammed as a priority.
When I bought mine, the seller (a private dealership for luxury cars), had just been broken into, and his safe with all his spare keys had been stolen.
The seller wasnt too bothered as all his cars are kept in barns etc, and viewing is by appointment only. So when a car is to be viewed he brings it to the office - otherwise they are kept offsite.
Anyway, I still purchased the car since he had promised me a new key was on the way and was paid for via his insurance.
I took the car as intended, since it would be kept in a locked garage when not in use.
The key took a while to arrive, since it came directly from Germany as the car was only a few months old.
As soon as i got the second key, I took it in to VW, and had both keys reprogarmmed - again the seller picked up the bill as it went through his insurance.
Hence Im unsure as to how much the key was, or how much it was to reprogram them, as the seller got the invoices directly to himself. But dont be put off not buying the car. Try to knock the seller down on price due to the missing second key, or get him to order you one, and see proof of the order before you do the deal.
Either way, get down to VW and get the key/s reprogrammed.