Colony Cost Reduction is a great idea, but one that doesn't really work as a tech. On a 0.0 planet it does nothing, whereas on a 2.0 world the increase is pop is so minor as to be barely noticeable. The place it really comes into its own is on a 100.0 cost world or so - a place that's such a bad choice for colonization that it needs some major balancing factor - ancient alien ruins probably, since even a treasure trove of TNE could be harvested by automines without bothering to put any colonists down.
So while it is technically a benefit, it's not something I'd ever choose to research, unless I was trapped in a single system with few or no ideal worlds.
I don't think the 10% is a percentage of the cost, but the percentage point that the cost drops. IE, a colony cost of 2, with a 10% drop, leaves 1.9. It's dropping 10% of 100 (.1), not 10% of 2 (.2).
There is a habability index for all planets. If this index is zero then there is no cost to colonize the planet and this tech will have no effect. If however the temprature is to high or low for your race, or there is a problem with the atmosphere then the index is used as a basis for how much infrastructure is required to support your population. The colonization cost is a modifier on this amount. The first level is a 10% modifier so having a planet with an atmosphere that does not have enough oxygen but is the right temprature would normally have an index of 2. With this it would be 1.8 (ie 10% less).
To really see how this would work save your current game database and experiment. find several colony sites with different index's, make a colony on them and then use the instant tech (Pop and Production screen F2 reasearch tab while being in SM mode) to give yourself a couple of levels of this tech. Then look at the summary tab and you will see how much infrastructure is required per million population.