AMEs professional fees rarely exceed £30 (before tax and NI) per examination - the rest goes to the CAA - you can insist upon a detailed breakdown on your invoice.
Take into account that the AME is obliged to attend various mandatory conferences and undertake a revalidation of their "licences" to undertake the examination every year and it adds up to a tidy sum which they have to fork out before they make anything.
By all means ask around, but I think you'll find little difference in what each AME charges for ther own time.