Simply put: Client A accounting records consist of 5 bits of paper in an envelope. Client B has well written up cashbook supported by an organised file of invoiced, a duplicate sales book and a sheaf of bank statements. Client A gets charged 150 quid because it takes 5 mins. Client B pays 500 because it tkes longer to assimilate all that info. do a Sales leder control etc.
Why not charge 'em both 500 - otherwise there is no incentive to improve. They are probably both doing the same job and earning the same money.