I would consider paying £100 for a scart lead if I had a £3500 plasma. After all, would you spend £1M on a house and then furnish it with DFS sofas? OK, a £10K sofa might not feel any more comfy than a £500 sofa, opinions would differ on which looks better, and in some cases a £10K sofa may not even wear as well. So, you would buy the £10K sofa provided you could identify it as definitely superior to the £500 sofa.
My scientist tendencies would push through and I would not buy that £100 scart lead until I had seen a diagram illustrating it's frequency response and screening, and would choose a lead from a selection of many rather than just buying it because some bloke in the shop says it is good. If a company isn't willing to publish detailed characteristics of a cable, then their cable can't be worth it! Would you buy a sports car where they refused to tell you the power, torque, 0-60 times or top speed? Especially if it cost 10 times what a typical family car does?
There are undoubtedly some very very good leads out there that on paper do the job far better than a budget lead (whether or not you would actually appreciate the benefit is another question
), but I would be very surprised if there are not some unscrupulous types selling c**p leads for big bucks. What is sad is some people will buy such leads and think "Oooh yes, I can see the improvement, much better!"