That's a reasonable kind of cost, I like that.
I'm still wondering about just making it a cloaked colony ship instead though. Makes you have to actually work harder (or in different ways, I guess) for your ideal planet-to-capture. I mean, odds are if you're neutering planets, you're knocking out the tachyon guardians, so you should be able to slip a cloaked colony ship through. Difference is, you don't need to bring cloaker starship escorts along in the case of a very hostile environment.
On the other hand, a Colony Starship would suffer from, say, Orbital Mass Driver fire.