Comparisons are always funny. I mean if you're going to compare with the sale of a young talent that hasn't been able to make the grade at his current club, surely Odegaard is a more like for like comparison...
It could be argued that with Palmer Chelsea are paying a 'hope' premium, while with Ode and ESR their true value has been depressed.
Better to look at each transfer on it's own merit, and on that basis, I think ESR is easily worth £30m+, and that's with injuries factored in. Otherwise as Qs says we may as well keep him.