We're talking about two entirely different leagues and cultures. The qualities you pay for with Henderson, suit this league. The ones you pay for with Isco, are based on what he's doing and how he's performed and is currently performing in Spain.
Henderson's worth over £20m in this league because he's developed into an integral player for us, from what was initially a promising, if arguably overpriced beginning. He's athletic, workman-like but has enough about him to play in a passing side that is fast, aggressive and sharp in the opposition half. That might not be something a Spanish team would be looking for, but it's something that's perfect for this league at this moment in time. Isco has qualities that see him as one of the brightest young midfield talents in Europe. He might go on to be one of the best, he might step onto English soil one day and be about as useful as what Arsenal picked up in the Summer.
Like I said, the market is distorted and however much player X's value has an effect on player Y's value, there are so many other factors to weigh in. A £20m player today can easily be a £10m player next Summer, or a £40m player. A players value to one footballing culture is dependent on his value to that environment. We judge him as a £20m player (or whatever), because his current profile dictates as such in that country. Regardless of comparative quality between (for example) Isco and Hendo, Henderson's value is the same because it's what he's valued at to this team and this league. Whether that translates to another footballing nation should he one day move there, is when we find out if his club market value is an accurate picture of what he offers the European market. His market value, should Chelsea or City want to buy him, could easily be £25m, because of the factors mentioned. Whether they would is irrelevant really, if one of them showed interest tomorrow, that's the type of fee we're talking about. Look at the valuations we were giving City over Skrtel and Agger.
Saying "why would teams pay more for Henderson" is ignoring the fact that we have. Other teams who paid similar fees for other players when they could have, in theory, bought better players from the continent. Has James Milner been a worse purchase than Ozil? It doesn't teach us anything new, just that there's alot of bullshit when it comes to player valuations, but we knew that already.