Pics would help, all I can tell you is from the info you've given so far. Missing info would be born date, size (45 or 68 typically), condition.
Carleton brand tanks aren't as sought after as other brands such as Crossfire, Ninja, etc. It should say somewhere on the tank when it was BORN and when it's last HYDRO-TEST was. Odds are it's carbon fiber and HPA (high pressure), so it'll have a 15 year max lifespan. The more years it has left in it's lifespan, the more it'll be worth.
Within that lifespan, the tank has to be Rehydroed (tested) every 5 years. This is the big factor in price because it's illegal to fill an out of date tank and costs $20-$30 to have it tested.
If in fact it has been tested in 2012, it should have 3 more years of hydro left in it. Now depending on it's condition (if there are any cracks or dents in it), it'll be worth from $70-$90