Whichever way you go it'll probably be wrong. It'll either be "how much!" if you stick to your guns or the good old "I thought it would be a lot more expensive" if you short change yourself.
Was going to ask you what you where thinking but its open forum.
out of interest how much would you charge to plumb a washing machine in, you've got the cold mains,so you need,w/mc tap, the waste kit and to core1"1/4 to drain,by the way it's on a out side wall and drain is directly opposite w/mc