Owing to the economy, you're going to see a wide range of prices. You'll also see a product shift its price by huge percentages at the same retailer. Transitioning between model years also contributes to this effect (particularly with last year's models of course). To take an extreme case, Pioneer KUROs can literally vary a thousands dollars or more between retailers or between weeks.
I can only answer a few of your questions, but I should point out that despite Samsung's attempt to saturate every possible market niche (there are sound economic reasons for this, though not necessarily in every market), most punters need consider only a few of the models.
In my case, I couldn't justify buying a B750, owing to its price. PQ is supposedly better than the B600s, and I prefer grey (sorry, charcoal) for my electronics to red any day. On the other hand, I'm sceptical about 240 Hz. And I wasn't going to pay 50 per cent more for somewhat better PQ -- at least, not for my ex-girlfriend.
Despite the fact I don't particularly care for the artificial look of AMP, I do
require 120 Hz in order to achieve smooth playback of 24 fps film content from Blu-ray without resorting to 3:2 pulldown. If you're going to switch off AMP anyhow, you should consider what sort of content you'll be viewing. If you're a cinephile who prefers the original look of film, then you'll probably want 120 Hz.
The 550s are 60 Hz only, the 750s too costly, so that leaves the 600s. The B630's screen isn't as glossy as the 'Ultra Clear' panel of the 650, so that's what I went for (ex-girlfriend has fairly well-lit room), though I do wish Samsung could have skipped their ToC and chin on the B630 (the A630 on the other hand looked comparatively elegant). Note that there do not appear to be non-glossy Samsung models above the B630.
I can't really weigh in on the input lag, as my ex-girlfriend isn't a twitch gamer (neither am I, actually). PVA panels (eg
Samsung and Sony) are usually slower than IPS panels (LG, Panasonic), but there's a considerable variation amongst PVA displays as well. Switching to 'Game' mode (which generally disables most picture processing) can sometimes improve the situation, though no one seems to find Samsungs, at least the B600s, to be particularly 'fast' displays.