Yeah, it is pricey.
I haven't tried Dior's version, or any Chanel foundation so I can't compare.
This is a recent splurge for me. Since it's so $$$, I think a couple things are important.
-Getting a perfect shade match, which can be done if you can find a store in your area that carries it. If that's not possible, use the website's shade matcher and order from someplace that has a good return policy.
-Application!! Let me tell you, it is really scary the first time you spray it on, but if you follow the instructions it will turn out fine, and by the 3rd or 4th time using it, you will feel like a pro! Now I think it's easier to use than my liquid foundations, because you just buff it...no brushes or fingers.
What makes it worth it IMHO is the finish and the WEAR. I live in the South where it is hot and humid and this stuff lasts and lasts without smudging, guards against oilies better than anything, and is a really good base (ie: my blush lasts forever too). This is what I wear when I know I have to look good for hours and I can't get to a mirror to touch up. 8)
I wish I could tell you how long a bottle lasts, but I haven't had mine for long. Using it every day, I presume it would last 3-4 months but that is a total guess. It can also be used as a body foundation which of course would make it last a much shorter time.
Does that help any?