First, curiosity and the need for understanding the universe and our place in it are real human needs. I agree they do not have the short-term urgency and fulfilling them is a long-term objective.
Second, having all of humanity live on a single speck of rock puts us in a very precarious position as a species and introduces a single-point of failure to our civilization. Therefore, the development of technical capabilities to move freely around the solar system and eventually beyond is also an objective for humanity. Again, I admit, it is a long-term one without short-term urgency of say, getting rid of Ebola.
So the whole issue is essentially a question of the right balance of the amount of effort we invest in working on our short-term urgent objectives and the long-term ones.
With world GDP in excess of $70 trillion per year [1], spending 1 billion EUR ($1.25 billion) for a decade-long mission does not seem like extravagance.
[1] http://en.wikipedia.org/wiki/Gross_world_product
EDIT: To further put the cost of the mission into perspective: assuming the contributions for the mission came from about half a billion people (EU population) over a decade this translates into about 20 euro-cents per year. Seriously, this is not extravagance. In fact, given that the mission cost is over 20% of ESA's yearly budget it makes me think Europe is under-investing in its space capabilities and scientific research. Nitpick: ESA member states don't overlap with the EU exactly, but it is a good enough proxy. More accurate figure would allow for American and other contributions to the on-board instruments so would be even less than 20 euro-cents/year/person.