I agree about the hidden costs.. i would consider the following when choosing and au pair and how you progress when you have one..
meals.. special dietary needs or needs different to your own. current ap is on a low carb diet, so eats allot o meat (expensive) wont eat the cheap meals I make like pasta and sauce or j. potatoes. previous ap only ate chicken, another drank gallons of coke. Ap doesn't tell me if she is in or out fro dinner, so food often wasted or not enough = grumpy husband.
electric and internet, ap's love the net.. we have had to up ours alot. we ahve solar panels and (in aus) yield power during the day that we earn more for then it costs us to use at night.. so washing machine etc should go on at night.. ap will not do this!
car insurance and accidents, be wary! it costs allot of money if you have to pay excess for a young driver.
water for us, we have tank water and ap uses huge amount in long showers.. expensive.
mobile phone provision. in addition to wages, presents at xmas, birthdays etc.
fuel in the car (we are soft and provide this)