I have a 43" Panasonic plasma TV, which has THX certified display panel. Paid about 800USD for it back 2013 I believe. I'd like to replace it with a LCD one, but oh boy how much do the THX certified ones cost - https://www.thx.com/product-finder/
Considering all but 3 of the 4k certified panels are Panasonic I think you're looking at the cost of panels from Panasonic that pay for the particular certification rather than looking at the cost of high end panels overall.
THX is just for sound, right? Why not buy a cheaper TV and a separate certified speaker set?
They have certifications for full home theatre including video (panels & projectors) but you're paying more for the THX logo than for a guarantee you actually bought the best option for the price since hardly anybody pays for the cert.
What does THX certified actually do for you on such a small screen?