I had 5 mins so I had a read of the thread just to find out what you're on about. I think you're both arguing with each other while not actually arguing with each other - running parallel and never even seeing each other. I'll give a quick shot at the main point of contention (while ignoring the childishness) and maybe correct me if either of you think I'm wrong.
Daru's side: If item A has a largely agreed value (let's say RRP to keep it simple) of £1000 and at some point goes on sale for £1500, it'd be common to call it overpriced - no?
Greg's side: Item A performs the job required and customer A is willing to pay the £1500 tag - therefore it's not overpriced because someone is willing to pay.
So you're really just arguing about what "overpriced" actually means. My personal take is that just because someone is willing to pay inflated prices doesn't mean it's not overpriced, but admittedly that's coming from someone who's reasonably poor so value for money (price : performance) is important to me. I can see Greg's side that an object's value is whatever someone is willing to pay for it, so if people are paying then it's not overpriced -- I just don't happen to agree with the sentiment (were I to have more 0's on my bank balance, I'm sure my sentiment would change)
Tl;dr - You're both right, you just come from different perspectives - arguing it won't change either of your minds but perhaps you can at least see each other's point, even if you don't agree.