I can't speak to the added price of 3D on 4K tv sets (I don't have one yet). But I can tell you that I spent a week with an LG 65" OLED 4K tv with 3D - and the 3D was incredible. (With 4K you get 1080p in each eye).
Also - there is a difference on which kind you are watching - passive or active 3D - I prefer active to passive as it is more enveloping. (Passive is what theaters use).
Not sure how far off glasses-free 3D is - but I do know that you can't fight the coming wave 3D and VR.
(I believe the TV manufacturers are just hedging their bets on glasses-free 3D coming sooner rather than later).