People who have a regular cleaner – how much does it cost and is it worth it? Do you just give them a key to the house and let them get on with it?

We have a baby arriving soon and both work quite demanding jobs, so for the first time in my life I am considering getting a regular cleaner. It seems like the websites of cleaning companies are totally useless at imparting any information beyond setting up a visit, but I would am an anxious millennial who would like to understand the process a bit more before I've got someone looking around my house.