A few points:
First, I think this post is better suited to the general led forum rather than the pricing sub forum.
Second: Which one? Several manufacturers now make or have announced a 65" 4K set.
Third: what is your current situation and time frame? How urgent is the need for a new set? I ask because as you may know the hdmi 2.0 specification (a safer future proofing standard when it comes to 4K) was just announced and TV sets which included it will be rolling out in the coming months.
Personally, if I were hell bent on a 4K set i would wait until sets ship with hdmi 2.0. (Note that some mfrs are saying they will perform some combination of a software and/or hardware update to give current 4K sets hdmi 2.0 but as a general rule I'm not fond of counting on updates in the field).
On the other hand, if extreme size is your goal at a (very) low price, I think it will be some time before 80" and larger 4K sets are shipping for the $4k of the current 75-80" sets. My guess would be two years though I could be wrong.
Anyway hope this helps.