have a look at it first, drive round it with him and say 'how much are they?' it sounds like he knows you enough to not nick the work, but a fair price on good work for that would be £1000-1500.
sometimes you have to speculate a bit, if they are underpriced and you would like more work, then work it for 6mths as is, and when the customers know you, then you can say "mrs smith, i'm really sorry but i can't clean for £5 anymore, they are really underpriced are these, and i need to put them up to £6.50-£7 from next clean". you will lose some but the ones you dont will balance it out and you will have the same money for less time, or more money for the same time.