I understand your logic, and I think most of your values are sensible on that premise (Yakubu's a bit too low, but whatever). But in reality, their value to us is higher than their value if they were at another club, because we know that they fit in.
For example, say we sold Arteta for £18m and bought, let's say, Moutinho for £18m as a replacement (just for the sake of argument, let's assume both players are equally talented). What if Moutinho doesn't like the English lifestyle? What if he thinks Cahill is a cock and it ruins the dressing room? What if he doesn't fit our formation? What if what if what if?
For that reason, the price we'd sell our players for should be higher than the price we'd pay for an equivalent player. The devil you know is better than the devil you don't, unless the latter is significantly cheaper, in which case we should sell our soul for the difference.
