The value of something is whatever someone else is willing to pay for it. It does not matter that something is not valuable to you - I can guarantee that if you priced the mission as low as $1m/$10m/$100m dollars, some PHD, wealthy person, or even SV software developer would personally pay for it - and there would be no regrets.

If NASA had to personally crowdfund these $5b missions, they'd probably still happen. That sum is also in the ballpark of Apple's marketing budget - I would not at all be surprised if they spent that much just to get their logo on the mission. Not to be outdone, Samsung pays next one.

But maybe you wanted to talk about the value-add to the society instead - which is easily going to dwarf $1m by many orders of magnitude just through advances in sciences, industry, and international cooperation that happen in the course of planning, building, executing, and pouring over results of such a mission. All of these things have a tendency to accrue interest, so the sooner, the better.

You could also take a state-centric view, and there also I guarantee you that the value of the image-lift and international cooperation is going to be worth more than a measly $1m to the US. A US president making a single international state visit costs multitudes of that.