Take one look at the new 4k displays and TVs hitting the market. CES this year, 4k was the big 'deal' as it seems the manufacturers have realized most of us could care less about 3d. 4k TV displays from LG were shown at CES @ 55 and 65". No prices given but assumed at around $10k or just under. Their 84" went on sale last year for $20k. Sony's new 84" XBR is $23.5-$25,000. Panasonic is also offering an 80+" TV @ right a out $30,000. Big screens. Big. HUGE pricing right now. When they start cutting smaller panels and content starts to trickle in (a 4k movie can be 150-200GB!), I think we'll maybe see some of the bigger AIO and separate displays of larger (+15") varieties start to pop up. Those of us with rMBPs know the software and sites still set up for lower resolution. They're making progress but the 15" rMBP is still only half of the resolution of 4k, and what would be viable in the 27" iMac.
IN other words, I think the poster you quoted about a thousand or two 4k tax is very low...at this time, with today's technology, it could be significantly more!