You're very correct the grinders are pretty pricey. I believe we paid about $300 for ours.
There are a few good reasons for me to have a grinder. Whether they are good reasons for you is your call.
1- I can grind whatever I want. Right now I am using hard white wheat. Unbleached, hard, white wheat flour is more expensive than your run-of-the-mill flour and comes in annoyingly small bags. I also grind beans, quinoa, oats, etc.
2- I can control the fineness of the grind. This is a minor thing but it is nice for me to be able to experiment with the texture of the product. In practice I usually leave it on the same setting but it is finer than the flour I can purchase.
3- Wheat flour is very perishable. I go through a fair bit of flour. As much as 200 oz in a heavy week. In order to have enough wheat flour on hand for a couple weeks it would have to live in the freezer. I don't want to waste that much freezer space. Whole wheat berries last almost indefinitely.
4- I like the flavor better. It tastes fresher and more nutty. This may be imagined- I haven't done any double blind tests. I should do that.
5- Cost- I had to do some of the maths as I don't usually pay a lot of attention to this.
King Arthur white wheat flour costs approximately $1.00 a lbs.
I buy 25lbs bags of bulk white wheat for $12-$15 or $0.50-$0.60 a pound.
If I use around 10 lbs of flour a week (usually a little less, it varies) I am saving about $5 a week. If you don't have access to wheat that cheap or don't make that much bread then of course the savings will be less.
As for nutrition- I read all the time that the nutrients in whole wheat flour degrade very quickly. I'm not a chemist but those results seem plausible to me based on how quickly the flour itself degrades in quality.