So one of the first things to remember is that the money isn’t exactly fired off into space, most of it is spent on engineers/scientists and the money makes its way back into the economy.
We also get indirect returns in the form of new technologies that are developed as part of the mission, these then make their way into normal use. A great example of this are solar panels. These were mainly developed so that we could have a source of power for spaceships, but now you see them everywhere.
Then we get long term, kind of indistinct returns. Developing the technology and doing a mission to Venus like this (or really any major mission) is going to move our species closer to being a multi-planetary civilization. This is something that is still probably beyond our lifetimes, but it is a crucial step in ensuring that we aren’t wiped out by some random event (like an asteroid). Each of these missions is bringing us closer.
All these reasons I’ve given can be applied to almost any space mission. But for this case specifically, the prospect of finding life elsewhere in our solar system is so exciting because it is the first time we will have ever found something else like this. It shouldn’t be thought of as something that will give a return, but rather as something that will help us to understand the universe better.
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