Players valuations (book asset value) are usually written down over the length of their contract. There may be a revaluation if a serious injury forces a retirement, for example. You wouldn't want to revalue upwards mid-term or you might be liable to a tax on the notional profit, unless you had made a big loss elsewhere and wanted to offset it somewhat, perhaps. I suspect this may explain the delay in tying some players down to linger contracts as there book values would probably increase.