We don’t live in the 1950s or the 1890s any more. We are not willing to sacrifice the lives of our explorers like we did when we went to the moon or Antarctica in the 1960s and 1900s respectively. We are not engaging in juvenile races to “get there first” which is both expensive and dangerous.
The space exploration of today is more collaborative and careful then the explorations of the past. So we don’t risk the lives of our explorers nor the unnecessary expenses of getting things done 20 years when we can do it in 50 for far less money and with infinity more safety.
So it’s not about trying to race and and get there first, but rather making sure that the opportunity doesn’t pass us by.
As far as cost goes… these sorts of missions are expensive relative to the amounts of money most of us work with in a regular basis, but compared to the vast sums that get put toward far more questionable and frivolous uses it’s a drop in the bucket. If saving money or rerouting funds to more deserving causes is a goal, there’s several tens of bushels of lower hanging fruit elsewhere that should be looked at first — anything with scientific purpose should be trimmed last.
What we learn there can be helpful for understanding the history of the solar system as well as planetary dynamics (remember that in terms of well-studied planets, we’re currently at sample size = 1).