Typically on United, cash upgrade is an international flight segment option only, as on domestic flights, plenty of elites have upgrade rights. Also on United, V class is not upgradeable on international tickets with miles, you'd need at least an H fare (although the good news is that there is no charge beyond the miles if there is availability). So, again on United, you'd be talking about upgrading from your U.S. gateway onwards.

If you're on American, my impression is it is miles (30,000?) + $250 each way.

Don't know about Northwest, might well be the same rules as Continental.

All of these only apply if you are actually on a plane operated by the company who issued your ticket. Some United flight numbers, for example, are on ANA aircraft and no upgrades are typically available on those without paying the entire fare difference.

There is no realistic hope of getting this upgrade for $100-200 or under any circumstances on a non-U.S. carrier.

For specifics, you can start by checking stickies at the top of the forum for the airline you are flying on.