Usually "habitable world" refers to a planet with CC=0.0 or very close. CC=2.0 or so is not too uncommon, you may discover an average of one every 1-2 systems depending on RNG.
What is rare in practice is not low-cost worlds but rather low-cost worlds that are also in good locations worth investment. By this I mean not only on a planet which has good minerals and is easy to colonize, but also the rest of the system has good prospects. Any CC=2.0 world in a backwater system can be used as a small fleet outpost with a dozen or so maintenance facilities, periodic MSP or mineral shipments, and a cruiser or two on station (and with Raiders in the game this is an important role to play). However, looking for a CC=2.0 world with good minerals, easy to terraform to CC=0.0, a rich asteroid belt, a nearby gas giant with accessible sorium, etc. is much more rare and these systems are the valuable ones.
If you want colonizable bodies to be less frequent I recommend reducing the gravity range for your species. By default humans have a gravity range of 1.0 ± 0.9 G, if you reduce the range to ±0.65 for example then these CC=2.0 worlds become more valuable, and you also start to face challenges where you need a fleet base somewhere but must use LG infrastructure to colonize the most suitable body. You can probably similarly limit the range of habitable worlds by changing your species' oxygen, temperature, or pressure ranges but I think messing with gravity is more fun personally.