Minimum release clauses are much rarer than many on here seem to think. Spanish clubs often have 'buy out' clauses, which aren't quite the same thing and which are usually pitched at astronomical levels and are totally unrealistic, but, in general, a release clause is fairly unusual. For good reasons, we could pitch it at £20m and undervalue the asset after a good period of form, or we could pitch it at a higher figure and basically scare off any potential suitors. Having nothing set in the contract allows the club to only deal when they want to deal, and at the level they are prepared to accept, not when they are forced to.