It's not like there's no position between 'we have to sell to buy' and 'sure we can afford to piss £25m up against a wall holding a discontented star to the final year of his contract'.
As I said in the 'How much should we pay?' thread, if we can afford to pig-headedly throw away £20m+ on Robin's transfer value now, why not just offer him an extra £50-100k/w on his 3-4 year contract, which would cost us less and quite possibly keep him on the books?
If the squad has to be sustained at its current level to keep us in contention, then if:
V is the expected financial value* of having Van Persie for the forthcoming season
X is the outlay to re-sign or maintain Van Persie
Y is the outlay to sign alternatives
Z is the return from selling him
Then as long as X < (Y - Z) < V we should definitely spend X.
It's an argument that may well have even more substance next summer if Robin is held to his contract rather than re-signing, because Z → 0 in summer 2013. Of course, his form and fitness may also decline (V → 0, Z → 0, Y → ?) …
… and then of course, if you've signed him to an exorbitant contract and he gets lazy, breaks both legs, or loses his edge, you end up with V < X over time …
- The distribution of V is pretty wonky, mainly based on whether we do/don't qualify for the CL