Join Date: Jan 2008
Location: Beaverton, OR
Bikes: Kona Jake, Kona Jake the Snake, Kona Major Jake, Kona Kula, Ridley Excalibur, LeMond Buenos Aires, Pinarello Gran Turismo, Pinarello Montello, Trek 614, Austro-Daimler Team, Motobecane Grand Record, Schwinn Voyageur, Nishiki Olympic, Steyr Clubman
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Look at the shifters. If they have a little red marker indicating which gear you're in they're 4500 series Tiagra. If they have a visible bolt, they're 4400 series. The 4500 series was introduced in 2007, I think, and is just being replaced this year. Those were pretty good components. If the bike isn't used, then the age really doesn't matter a lot. The Tiagra components on a 2007 bike and a 2011 bike would be the same.
You might be able to figure out the exact year using Bikepedia. Color and components are usually enough to figure it out. Is it light blue? It seems that 2007 is the only year that C'dale used "Optimo" int the bike names, and the 2007 Cyclocross Optimo 3
had Tiagra components with a 105 rear derailleur. If that's the bike you're looking at, I think it's probably pretty good. You could find something used for a bit cheaper, but it would be used. The 2006 C'dale Cyclocross also had a Tiagra/105 mix, but it had the 4400 series shifters.
Edit: I just re-read your post and saw that you do say that it's used. Given that, the price isn't amazing, but it's in the right ballpark.