Of course the amount is relevant.They cant just decide to charge 'x' number of pounds.The charges have to be proportionate to their costs.Under the Unfair Terms in Consumer Contract Regulations the charge should not exceed the cost to the provider.
Obviously the Banks disagree and thats why there is an ongoing Court Case which the Banks are now taking to The House of Lords.
Indeed, the unfair terms in consumer contract regulations does not apply to service or product charges, only to penalty clauses, so the banks are right to keep going with this objection.
I'm still waiting for the moans about the unintended consequences if the banks do lose the ability to charge customers who insist on spending money that is not theirs... The approaches that will be taken to stop that occuring will not be pretty.