I don't know where the actual line is for price vs performance but I feel my experience in speakers can maybe help some out. Let me start out by saying that I've enjoyed every speaker on this list and the best advice I can give is to enjoy the speakers you own,
I started with some old Polk Rti100 powered towers, which were actually pretty good, I "upgraded" to the Polk Monitor 60s which were most likely a downgrade but I didn't know it at the time and didn't have the measurement software and expertise to dial in the powered sub.
After that I tried the ARX A1b, now Chane, bookshelf speakers. Very surprising to me, they sounded very neutral and clear compared to my Monitor 60s, they didn't match the bass output but it didn't matter since I didn't listen that loud and now had a subwoofer to handle the bass.
I tried a few speakers in the next few years including the Def Tech SM45 and SVS prime bookshelf, neither made me want to change even though the SM45 were very good.
The Wharfedale Diamond 220 is what finally made me upgrade from the Chane A1 after a few years, they were actually very close in sound quality but the Diamonds were slightly better and looked better in my opinion.
After this I wanted to try something with a RAAL tweeter and since I had dual subs that I could equalize flat with any speaker, I went with the Min Philharmonitor. I wanted to compare the RAAL to a dome tweeter so I also built the Zaph ZA 5.2, which is the same woofer and a Vifa dome tweeter. The RAAL was everything people said but to my surprise, I actually preferred the ZA 5.2 and its wider dispersion.
That didn't last long though and I borrowed a pair of LS50's from my dealer and they blew my mind. The highs were mildly fatiguing so I ended up buying a pair of Q150's and using them for about 3 months but I kept thinking I was missing something compared to the LS50's. I grabbed another pair of LS50 to compare and head to head they easily beat the Q150, mostly due to the highs above 1khz.
I've tried the Polk LsiM 703, Philharmonic BMR and Revel M105 since then, the Polks and BMR are both great speakers that don't really need subwoofers to sound great but the M105 was the closest speaker to the LS50 in my opinion. It's actually the speaker that made me have to perform a blind, level-matched, mono test (Harman style) to pick a winner but I still chose the LS50. The M105 is my 2nd best bookshelf speaker under 2k though.
I've listened to quite a few speakers and have had the LS50 for over a year now but to your original question, I don't know if I have an exact dollar amount of when diminishing returns sets in but I would say in 2019 if you can find the KEF Q150 for $299 you're not really going to beat it until you can find the Polk Lsim 703 for 1/2 price, and that's only if you're not using a sub. For 2 Channel use, the 703's are tough to beat and the build quality is excellent. If you go above the $1k range and have a sub(or preferably 2), I personally haven't found anything that beats the LS50 but I'm sure others may disagree.