How much "more" for how much more $ ?

While there are far too many variables involved to be able to generalize meaningfully, I would say that the extra $9K will often go towards a larger cabinet, perhaps more drivers, and perhaps a larger woofer or woofers.

All of which will mean deeper bass response, perhaps greater sensitivity, and perhaps greater ease handling high volume peaks. In turn eliminating the need for a separate subwoofer in many cases, and in some cases also reducing the necessary amplifier power, and hence the cost that has to be paid for an amplifier that is otherwise similar.

Certainly there will often be significant sonic differences as well, within the limits of each speaker's frequency range and dynamic capabilities, but as I said I don't think it's possible to generalize about them in a meaningful way.

Regards,
-- Al

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.