Before 6.0, I managed to reach 1.9 colonization cost for Titan after a lot of terraformation. Now that the lower limit for human beings have been changed from 0 to -10, the colony cost will be even lower.
Sure, Titan will always require some infrastructure, but it is more of a boon than a pain : the civilian sector of your habitable planets will supply a lot of those, and the civilian transport will boom thanks to it. My current Titan is inhabited, not yet terraformed (Jupiter's gallilean sattelites were far better targets) and is taking all the infrastructure of the 7 others inhabited (and far more pleasant climate-wise) bodies of the solar system (Earth, Moon, Mercury, Mars, Europa, Ganymede and Callisto. Not Io. Stay away from Io they said.)