You might want to look again at what is being sold with it. The mech itself is not all that special aside from being hard to find pulling it from a booster. If it is being sold with "original pilot and gear" you might have the reason for it's higher value.
Name: C3i
ID: DOM-G-117
Expansion: Domination
Points: 17
Class: Light
Faction:
Unit Type: Gear
Attaches to: Attack
You may have one C3i gear in your battleforce for every 150 points of the game's build total. When this unit makes a ranged combat attack against a single target opposing figure, it gets +1 (to a maximum of +3) to its attack value for each other figure friendly to it that is also assigned C3i gear and that also has clear line of fire to the target and could make a ranged combat attack against it.
It is possible that this gear card is being included in the sale, a set of three or four of these can cost more than the mechs they came with. This is what made the Dominion unique mechs so desireable, though the gear can be used with any faction.
DOM-G-117 C3i (Light 17 pts.)
DOM-G-118 C3i (Medium 26 pts.)
DOM-G-119 C3i (Medium 26 pts.)
DOM-G-120 C3i (Medium 26 pts.)
DOM-G-121 C3i (Heavy 33 pts.)
As you see, there is only one version of this gear in light and heavy weight classes. Four light mechs can be built into a 600 point tournament army that can be hard to stop if they are using this gear. It's not as useful for larger mechs, as they tend to deploy in large battles that exceed the 900 to 1500 point range.
Some players will deploy light mechs as if they were "cheap heavies", but I like to keep the cost low and have bunches of them and enhance the damage value rather than the attack skill.
GC