Cost is never immaterial. We look at the various ways we have to provide fresh water, and do whatever is cheapest. If shipping fresh water from places where it's abundant to places where it's scarce is cheapest, we do that. If/when that becomes expensive enough to make desalination attractive from a cost perspective, we start doing that. If the >230m below the surface idea turns out to be cheaper than surface level plants, we do that. But I don't think anyone really knows for sure if the underwater method could be cheaper, so we're all just guessing here.
I think a good analogy for this is oil extraction. These days we extract oil from places where it's more expensive and difficult to do so, because we can no longer meet demand with the cheaper options (many of which have simply dried up over time). But we'd never go after the expensive-to-extract oil 40 years ago (or whatever) when cheaper options were available that could meet demand.
Yes, economical is always important. In this case, we’re comparing a 231m deep unit with the “boring” existing design, and finding it less economical than the current designs.
It’s not just that it’s expensive, it’s also ridiculously complex. About the only thing I can think of that would be more complex is launching rockets into space in order to collect water from comets.
