On the basis of where this discussion is going why not just charge them £100 and do them once a year?
The fact is that customers are price resistant like we all are and the idea of handing over more money to get them cleaned less often doesn't look like the best deal in the world does it?
There is some advantage in the six week clean. My round is nearly all six weeks but you can't drive up prices too much. Instead you make a bit on the increased time efficiency of WFP and the fact that you get less tired so you can do more houses in a day.
In my area, the so called affluent London suburbs, you would think that prices are rock solid but the undercutters are always snapping at your heels. I lost a very long established customer yesterday to what I think was an undercutter. She said her husband has decided that he is going to do the windows from now on. Knowing her husband I don't believe that, so its either money problems or someone has tempted her away.
You can only go so far with prices, look to efficiency to make it more profitable.