Nobody is seriously looking for ‘habitable’ planets because they expect humanity will someday inhabit them — this is all about the hunt for other life out there in the universe.
To astronomers, “habitable” just means that the planet gets to correct amount of energy from its star that liquid water could potentially exist on its surface. Liquid water may not actually be a requirement for life, but since we only have a single data point to work from, it makes sense to look for the preconditions of the kind of life we’re familiar with on earth, of which liquid water is a big one. (Another is carbon chemistry, so finding lots of atmospheric carbon isn’t necessarily a bad thing when searching for other life out there.)
I did some quick back-of-the-envelope math, and unless I’m mistaken that’s equivalent to about $1 per gallon of gasoline… which is a lot lower than I had expected, honestly.
If that’s all it costs, we should be sinking billions in public funds into this, especially if there’s an R&D component that’s seeking to drive down costs/increase efficiency.