A standard semi with 10-12 windows will use approx 15l of water, but the bigger the tank you have, the less you are bothered about how much water you use.
Cost wise I guess that if you are on a meter then it will add a £1.00 a day to your water bill, though that will vary depending on what your water company charges.
If you put in a couple of weekends of practice, on your own home and friends and relatives then you should be able to develop a reasonable technique.
Better still attend one of the day long courses that are on offer.
You will still be pretty green for a few weeks, but your understanding of the techniques, of the problems that will arise and the solutions to them will be in place.
But 30 houses a day?
Thats 5 an hour done continuously over an 8 hour day, that is one hell of a work rate.
Ian