Unfortunately, with the discovery of the Juggys, the GTVA have very little choice but to invest in Colossus class ships, as they are the only (speedy) way to deal with a small incursion. From the GTVA's point of view, a lengthy engagement would be disastrous, as the complement of fighters and bombers carried by a Sathanas, over time, would easily overwhelm a star system.
Also, a Colossus class of ship makes a nice anchor point for a fleet. In terms of naval strategy, it also becomes a 'fleet in being', a bit like Bismarck (excuse spelling) in WW2. ('Fleet in being' is an extremely powerful ship or fleet that, if deployed, could cause serious damage to an enemy. The enemy is therefore forced to devote time and resources to find it and attack it, even if it does nothing.)
Regarding production and costs, it depends on how the Colossus was designed in the first place. In FS2, I suspect that it was to be a one of a kind trump card; but now has to be converted to mass production. Not a problem - we do it all of the time today. Think of the Nimitz class of carriers to see that it can be done.