What Sonically is the Difference between a $1,500 CD Player and a $10K-$25K One?

What no one seems to be talking about & is essential to realizing huge qualitative gains by spending a lot more money on digital - is if that money is put to excellent use in designing a truly outstanding analog stage. It’s far rarer then everyone likes to assume. I always was bored silly by the very detailed Berkely ref until they did a thorough redesign of it. If its a really well-designed tube one then you have something really special. Zanden did an excellent job here (although the digital section was slightly compromised by their choice of chip). Their new DAC & chip could very well be fabulous though. Micromega’s best tube Dac is also supposed to be wondrous. Some others too.

Just don’t listen to a quality record playing system of the same money as expensive digital. You'll become one of the many you see selling expensive digital gear because (as they say in their ads) they’re going back to vinyl. What you don’t know won’t (perceptibly) hurt you here.

All CD players throughout history have been plagued by fatal problems. While it may be true that some advances have been made in CD playback nobody has solved the problems of scattered laser light, seismic vibration, and the various vibrations produced by both the CD player and the CD itself. If you don’t mind the missing information, the crappy bass performance and the compressed sound, more power to you. Yes, I know what you’re thinking, “What missing information, crappy bass performance and compressed sound? My system sounds fabulous!” 🤗

These kind of questions are sadly just going to do no service to the person who is asking. I am sure you are asking in pure hearted curiosity. Unfortunately you are wading into a battlefield occupied by several armies.
There are certainly many CD players that are over priced but to say that all high priced players are flim flammery is to dismiss the costs of development, testing, parts quality, manufacturing, distribution, etc.
It is a complex question that can really be understood best by auditioning a lot of equipment yourself.
In my case, I have heard a lot of stuff in a lot of different settings to come to my purchasing decisions.
I would just advise setting a budget and listening to as much as you can (Never mind the price) then settling for what you feel hits the mark in your range.
For me, I ended buying something that was about a third more than my original budget because it just sounded worth the money.
You can read all of these posts and get a feel for all the current hardline attitudes and then make your decisions.
My decision has always been to trust my ears. Mostly they haven’t steered me wrong. If they do it is usually because I didn’t listen long enough or carefully enough without distractions.

I had a Bryston BCD-1($2,750) and found it superior to what i had heard before. It's shot and my limited funds have gone into analog playback instead. Though i would trade into the Bryston BCD-3, $3,750, ($2,200) with returned BCD-1. I heard a $15k disc player at a friend's which sounded far superior to the Bryston. Zero shortcomings,  very analog. I don't have a DAC, next time some money shows up I may dive deeper into analog by adding a second tonearm for a mono cartridge and phono stage. Leaving digital out of my listening room.

glupson - Correct that CDs don't fail very often, the laser is usually at fault. 1 or 2 out of 7,000 CDs made since 1984 have failed me. I make it a point to clean the laser every year (1,000 hours) or sooner. It makes a difference with 80+ minute CDs (London classical reissues sometimes go to 85 minutes).  
Elizabeth and roxy54 - I 100% agree.  The mastering is numero uno in the resulting quality of vinyl and CD.  Just last night, I played Turandot/Mehta on a London CD but when the chorus came in, it was a hashy mess.  I put the LP Decca version on and bliss, full, detailed sound.  (Why do so many of my London opera CDs sound like poor quality remastering compared to the LPs whereas London's mono classical sound superior to their LPs)?  
I went through about 40 used CD players from 1985 to 2000 (mostly average quality/$500-$1500 Denon, Sony, Yamaha, Marantz etc.) until I found the EAR Acute 1 in 2007.  Audio nirvana with NOS tubes and cabling.  My friend upgraded his CD only system with a COS Engineering DAC using a Pioneer Elite DV05 as a transport.  Yes, it's got superior resolution.  I do not have the problems geokaitt claims CDs have with well recorded and mastered CDs.  My sound engineered audioroom cost twice as much as my main audio system but it's worth it (only an external tweak here and there, no big change ever again).  I have friends who are CD or vinyl only listeners.  Funny how they both enjoy my quad  front end system (plus RR and 78s).   CDs are just easier to use than LPs.