How about a high desk that you can stand at, with a stool or drafting chair for when you want to sit? A tall chair seems a lot easier / cheaper than an adjustable desk.
In my experience ergonomic means with a too much bended back shape that will DISTORT(!) your natural spine shape. Unfortunately they use statistic to decide what is the ergonomic "best" shape, the problem is that people backs in the west have gotten less and less straight the same way they are getting more and more overweight.
Instead of buying an expensive chair, in my opinion it is a best investment to buy a good book or take some good posture lessons that teach you how to sit, stand or sleep properly.
Your back could support itself without effort if it is straight.
I was of the similar opinion, but a research[0] suggests that sitting straight is not actually as healthy. I'd like to see a source which would confirm that sitting back causes distortion to spine shape.
That said, I agree with you that an ergonomic chair would make little sense for a standing desk, as one will obviously sit less (only when too tired to stand).
I don't think it's obvious at all. I prefer to sit, but I am by nature a restless individual. A standing desk would allow me to make some of my pacing time productive.
To put it another way: every person may have a different use pattern when given the option of equal comfort at seated and standing heights.
A top-of-the-line aeron is about $700; the stool version is about $1200.