If that's not your claim then I don't understand your valuation and the rest of the comment doesn't track.
And not to engage at a base level but my use of immeasurable is correct in being interpreted as "large" unambiguously, at least by Merriam webster.
A "viable billion dollar mission" includes all sorts of other things that are of value, including developing capabilities to do things that are strategically important, which I would claim is where a large majority of that billion dollars comes from. Similarly, I would expect that JPL would very much inflate the value of their own work. Everyone does.
Also, I see "incapable of being measured" with "broadly : indefinitely extensive" tacked on in the MW definition. There is no requirement there that "immeasurable" mean "large", just "incapable of being measured" with the expectation that it is used when describing things that are extreme. My most recent use of "immeasurable" was "immeasurably small" which I'm sure you would agree is a proper use.
Hosting the photos costs money too, which I don't think GP is considering.
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.