Personally, I distrust the statistics. I don't think you can compare the cost of planned homebirths with that of planned hospital births.
Planned hospital births are always going to include a fair proportion of women like me, whose medical problems always made it clear that there was only one available plan. By the time I had my planned hospital birth, I had been spending several weeks on the ante-natal ward. And yes, people like me cost a lot more to deliver. So we push up the costs for the category of planned hospital births.
Also, a lot of the planned hospital births include first mothers, where medical problems may not be apparent until after they start labour: this happened to a fair few friends of mine. In other words, if they hadn't already been in the category of planned hospital births, they would have ended up being counted in the category of emergency births.
But I really don't see a reason why I should grudge other more fortunate women to have a happier, more natural experience of giving birth. For me, it was always going to be a fairly medical experience and I'm fine with that; doesn't mean everybody has to go through the same.
I also understand that there is a good possibility that some women are so stressed by being in hospital that it might make their labour longer (and consequently more expensive).
People are different, let's just accept each other.