Will you be using SVHS quality or standard Composite signal quality output from the PC?
A long lead lowers the picture and sound quality, it is reckoned you can get away with up to 30 feet when using standard composite signals before the loss it too bad, BUT you must use good thick well screened cable. I don't think SVHS will travel so well down a long cable and you'd need four wire flex for SVHS plus audio leads.
Assuming you find a shop selling GOOD quality cable I would hazard a guess of maybe a £1 a metre. Don't forget you need a twin pair screened flex for the stereo sound and a single screened flex for the video signal if composite, or 4 lead flex for SVHS.
Personally I'd find it too much bother and get a legal video sender for about £70 (Thomson or Vivanco etc) or buy a second DVD player if you're feeling flush!