200-400 USD
Assuming very good condition, a blue used Vaquero goues for $300.00 - $350.00 around here. I bought a pair of blue 5 1/2" vaquero's a few years back for $500.00 for the pair.
You will have to ask Ruger.
About $500
50-300 USD or so
100-400 USD or so
A 44 cal (magnum) is an 'original' Vaquero as opposed to the New Vaqueros which are only sold in 45 Colt and 357 Magnum. $400 will get you a decent, 'unimproved' example. It is still possible to find these new. Expect to pay a minimum of $500-575. Strangely, stainless models don't seem to command a higher price than basic blued models.
50-1000 USD or so depending on specifics
Depends on what shape its in but $250-$300 is a good price if its decent.
List price about $570, used price depends on condition, 100 pct $460, 98 pct $345, 95 pct $270 etc.
$200-$300, depending on exact model and condition.
1 dollar or less if you can find one. Otherwise 100-400 USD
Ruger's website has a sn function.