An Au Pair: Tempting Idea - But What About the Au Pair Costs?

Michael and I can simply see that we need to change something in our lives. It can't go on like this. I'm a boutique owner and he works in IT. We have two children. Mia is now 3 years old and Felix is 13 months. My parents can't manage it, and Michael's parents live 95 miles away. Somehow we both feel that they're happy to come and help, but not as regularly and not according to my requirements.
I need reliability, so I'm considering an au pair. In that context we're currently looking online to see what the au pair costs look like. We have the space. Our house has six rooms and a total of about 2,500 sq ft (230 sq m). There is room for an au pair. There would be no au pair costs for a separate apartment.
How High Are the Actual Au Pair Costs?
There are various examples online of roughly what the au pair costs look like. I'm a bit skeptical here. Sure, the usual insurance and the pocket money come into play. But in the end, an au pair is also a family member from the moment she arrives. If we go somewhere, the au pair comes along. The au pair costs would already challenge the monthly household budget. So far, however, we have had no childcare costs at all.
What advantages would we have:
- Independence from the grandparents' schedule
- the au pair lives in the house
- helps with the housework
- can take some organizational tasks off my plate
- the children may no longer have rotating babysitters
But how does it feel to have a stranger living in the household with us?

An Au Pair Is a Longer-Term Decision
What we want to avoid is having a revolving door of au pairs for the children. If we go this route, an au pair should come to us for at least 1.5 to 3 years. Is that even possible? We need a lot more information. We're finding that the deeper we get into the topic, the more questions come up.
It would be nice to get such relief, but might we end up with a third child on top of it? Who guarantees that it will work out then? The reports online vary widely, but most draw a positive conclusion.
The grandparents on both sides are rather skeptical. A total stranger in the house. They can't imagine it at all. Simply trying it out for a month just doesn't work. We'll sleep on it for many more nights, and yet the direction is already clear.