The way I see it is every job is not the same, so you can never really generalise on a price. Also a plumber in london will always charge more than a plumber out in the sticks. 18 months ago I did an ensuite bathroom with dressing room organised by some poncy london designer that had a budget of £50,000, last week I installed an ensuite for £200. so each job has to be looked at on its merits, the way I see it is this, you do a job that your proud to put yer name to, you do a job that your customer is pleased with, you do a job that leaves you with a reasonable profit, and you do a job that has no call backs.
regards
mike