So what happens is Bale was a 1 season wonder and fails to replicate his last season form?. This could be the only year he can potentially grab up to 60 million for 1 player. He could rebuild virtually every position they need for that money.
Tough choices at the top...
Whilst Bale certainly isn't a one season wonder, there's precedent to significant drops in value as you describe.
Two years ago Wayne Rooney was reportedly the target of various £60m bids from Europe's rich boys; today it's expected that a bid of under £30m will secure his signature.