In my pre-FIRE days I hosted au pairs. All together we had 5 au pairs, with two of them being excellent, one being good, and two were disasters.
The two bad ones were our first ones, and to be honest I didn't know how to properly interview them and such and to figure out a good fit for us. Of those two, one was truly a bad au pair, and one was just a bad fit for us. She went on to another family who were very happy with her.
Our au pairs (good/excellent) were all older, they were 25-26 when they came to us, with extensive child care experience, driving skills, and good conversational English. They were from Brazil and Colombia. One of them is now, 11 yrs later, our close friend. We still keep in touch one other.
As far as costs, we paid agency fee (around $8-9K if I remember correctly) plus $200/week stipend directly to au pair. We provided them with cells phones with reasonable plans, and obviously room and board. In our case, we had a room with a bathroom in our walkout basement where au pairs lived.
We had a car for au pair to use for kids and for herself on the weekends/evenings. We gave them gas money to cover gas needed to drive kids around to where we needed them. They had to pay for their own gas. Adding au pairs on our car insurance increased costs very minimally but I don't remember amounts.
I remember having to buy a winder coat for one of them when she first came in November because she didn't have any warm winter clothes. That year she saw snow for the first time in her life. I'm sure there have been some other costs along the way, birthday gifts, year end bonus, etc, I don't remember all of them. I think we gave them an extra week pay for Christmas and bday gifts in the $100 range. But they also made/bought gifts for my kids for their birthdays. One of them gave me a scarf for my birthday that I still love and use 8 yrs later. With good match au pairs it really can be a good situation for all, au pairs, kids, and parents.
My two excellent au pairs were with us for 2 years each, we happily extended with them for the second year. My good au pair was with me for her second year. She spend her 1st year with family in the DC area and that family did not extend with her. I grabbed her and she spend her second year with us. She was good au pair, responsible, did everything she was supposed with kids, etc. They just didn't bond as much with them as the two excellent ones did. But at the same time, kids were a bit older by then too so that may have been a factor. In any case, I had no complaints with her, but once her year was up we parted way and never stayed in touch.