Canon has recently launched new Canon 6dmk2 which certainly fit into the category of "Cheap" full frame DSLR. It is launched as successor for Canon 6d.
So If you are shooting with Cropped body and looking for upgrading the full frame and are tight on the budget, then new 6dmk2 will excite you. 4k timelapse movie, touch screen, dual pixel cmos AF , 6.5 FPS, In-built inbody stabilization and may such amazing new set of features make 6dmk2 an excellent value for money machine.
But if you compare it with much costlier full frames like 5dmk4 or even 5dmk3 (which is a a generation old) it lags behind by miles when it comes to some critical factors.
One of such factor is "Dynamic Range" which is significant when it comes to any genre of photography. Every now and then photographers are required to pull up the shadows and dump the highlights in the post. Canon 6dmk2 has significantly lower dynamic range.
Second important factor is cluster of AF points. Canon 6dmk2 has all its AF points concentrated in the center of the frame that will make photographers feel un-comfortable whole composing the shots.
So if you already have 5dmk3 then ... you still have Canon's most popular DSLR till date and it is for a reason. Keep it.
Its better not to compare 6dmk2 with one of the best from canon i.e. 5dmk4.
Bottom line ... 6dmk2 is CHEAP full frame option for APS-C owners but with some compromises. Do not expect it to be as good as the costlier ones in the same class.
Recently tested 6dmk2 ... so any questions are welcome.