In fact the problem is not the cost of the DVI cable but the cost of the DVI entry on the screen. In France, widescreens with both DVI and VGA entries are about 100 € (120$) more expensive than the ones with only VGA entry... this is due to a special tax (DVI monitors with a diagonal of more than 20" are considered as possible TV).
By the way, could someone answer my first question concerning the different resolutions in 16/10 aspect ratio?
Ouch... that's one damn ugly tax. :(
And The_cranky_hermit kinda answered the other part. It depends on the monitor and the graphics card on how different aspect ratios will be displayed on your monitor. The two main functions are i) stretching the picture over the entire monitor, or ii) using fixed aspect ratios only. A 4:3 resolution will look a bit messed up if you stretch it over a 16:10 monitor, of course. If you keep the correct aspect ratio of a 4:3 resolution on a 16:10 monitor, typically the picture will be shown in the centre and you'll have some black "bars" on the left and the right of the monitor.
Some wide-screen monitors have options for this, but some don't, which would mean that your graphics card has to deal with this. I'm not sure if ATi cards have options for aspect ratio scaling. NVidia cards do for sure. My monitor is 2 years old and it has options for this, so I guess most monitors nowadays do. But read the product details and read reviews on the Web before you buy your new monitor.