The lower-priced ones generally have to show-off a few large figures, so people will be attracted by them, or con them into thinking they're getting something more than they actually are. These bloated contrast ratio figures are not the true values, but merely 'dynamic'.
The more expensive units don't have a reason to quote the higher dynamic figure.
Having said that, unless it's a proven poor quality screen, all present TFT monitors' contrast ratios of 800:1 & above will be perfectly acceptable for most users. G