Hi Carolina2007 and welcome to the forums! I'm not sure who this question was directed to, but I thought you'd get better responses if I split it out into it's own thread.
I have a '94 Limited 34'. If the trailer has been sitting, you'll want to have new tires put on, inspect the brakes, and have the wheel bearings packed. Check for soft spots in the floor. That means rot and is not good (expensive to repair). Make sure that all of the appliances work. A new fridge can cost as much as $1600. Look for obvious signs of damage and seam leakage (water stains on drapes/upholstery/carpets etc). Check that the frame doesn't show an inappropriate amount of rust. The most important thing isn't the age, but the maintenance it's recieved. A new trailer that's been abused or neglected is a poorer investment than an older trailer that's been properly maintained.
The values will vary with age and condition, but generally run from around $10k for an older ('80s) 34' in good condition to upwards of $30k for a later '90s.
Past that, it's pretty much all cosmetic and what you're looking for in options and decorating.
AIR 2053 Current: 2004 Airstream Interstate "B-Van" T1N Sprinter & 2006 Born Free 32 RQ Kodiak Chassis
Former Airstreams: 1953 Flying Cloud, 1957 Overlander, 1961 Bambi, 1970 Safari Special, 1978 Argosy Minuet, 1985 325 Moho, 1994 Limited 34' Two-door, 1994 B190 "B-Van"