How much "more" for how much more $ ?

I would say it depends on the product and how you perceive the sound. Is the improvement meaningful to you and do you have the proper system to take advantage of a higher quality speaker?

I think the law of diminishing returns applies. Combined with room size, you theoretically reach a point where it takes a larger incremental investment to achieve smaller improvements. I would doubt that a $25,000 turntable is five times better (measurably)than a $5,000 turntable.You might argue that it gives five times more pleasure because of subjective factors like pride of ownership, etc.
Just my 3 cents worth.

Maybe people are tired of hearing this but... The best, and cheapest, way to get a speaker upgrade is to treat your room.

We, me included, sometimes, fall into a "reverse snobbism" in audio; the feeling that gear over a certain price is some how created to take advantage of someone's ignorance or status craving. it's too broad a generalization.

There are some great speakers - from small 2-ways to hybrid Horns - that start at <10K. There are also Hi-Fi detail death-rays, it depends on one's taste.

i concur with all of the above--an extra 9k will definitely buy you sonic improvement; you'll also get enhanced aesthetics and build quality. however, as drdennis states, these improvements definitely do not increase in proportion to your investment. that said, my personal take is that 5k is more than adequate for excellent speakers in an average room; i wouldn't spend more unless (a) the room was a dedicated music room and you listened at higher volumes; (b) your accompanying source, amp, etc. were commensurate in quality, and (c) aesthetics and other non-musical factors were a priority for you.

9K in a speaker may get you drivers costing several hundred dollars each as opposed to the prototypical $50 woofers. Although sometimes it just gets you a high quality veneer.