S.S.
THe official au pair program for foreign young ladies who come from other countries is $195 for 45 hrs of work. Then you also pay the agency fee $6-8K, and you pay up to $500 education expenses, for room & board, and of course there are some misc expenses that come up during the year. All in all, it's $20K+ family cost.
If paying by the hour, it is usually by # kids. $10 1 child, and up from there. I was paying $13.75 + taxes for our American sitter before we had our au pair.
Good luck!