I'm assuming the 21 refers to the number of gears. If that's the one it was $300 as new, so at most it'll be $100 today.
Really hard to tell. They have pretty much the whole range from entry-level bikes to high-end race bikes.
A Trek Bike.
It sold for about $450 when new, and it wasn't that special even back then. If it's in perfect condition-maybe $100.
It can cost anywhere from about $400 to $9,000.
trek
A Trek Madone bike can be located at retailers such as CS West Bikes, Outside Outfitters, or Pro Bike Supply. If you prefer shopping online you can check out eBay and Amazon or online retailers such as Trek Bikes, Bicycling, or Wimbike.
depends what your looking for really but yes it is a good bike
About £5 ($8.32)
maybe 50- 100, craigslist
Trek MSRP for the Series 3 Hardtail - $529.99
1987
You can purchase a Trek Domane 6 Series bike from your local bike shop. Head over to the official Trek website, and you can find the location that is near you.