Nope, it isn't like that at all.
This is why I said 'a number of factors' go into this stuff. I obviously need to elaborate, but one of the key factors that has gone into determining Lukaku's price, in addition to just Everton's reluctance to sell, is the fact that he's a proven consistent goalscorer. Which is far from the equivalent to a Toyota. The fact that he scores 20+ goals a season is a factor in determining his market value. The fact that he's just 23 is another factor.
Isco' value, in any reasonable market, is immediately diminished by the fact Real want rid of him. There are even two different prices for a Bentley that a man is keen to sell and one that he isn't, to use your own analogy.
Anthony Martial is a young player with huge potential. I can only imagine the difference between whatever price we would have quoted a buyer after his first season, and one we would quote now if we started putting feelers out into the market that he was available because Jose has decided to get rid.
Out of curiosity, every single factor considered, how much do you think Everton should ask any prospective buyer for their star striker? £30m £40m. In the interest of being 'fair' to the buyer.