So why is NASA spending $2.5B on the next Mars Rover and planning to spend over $6B more on a Mars sample return when it can't find the money for much cheaper missions to Europa or Enceladus?"
This summary doesn't accurately describe the situation at all. The Mars missions are so more expensive largely because they are doing more. The next Mars Rover is going to be larger, heavier, and more capable than the two previous--wildly successful--rovers in pretty much every way. That $6B mission is a sample return mission, lifting off and bringing a research payload from Mars back to Earth is an enormous technical challenge. It's never been done before and that will drive most of the cost.
Also the linked missions aren't quite as cheap as the summary implies. The proposed mission to Europa has an estimated cost of $2.5 billion (and $4.7 billion is the given estimate in the last paragraph of the first link in the summary), exactly the same price as the first "overly expensive" Mars mission mentioned. The Enceladus trip is much cheaper, estimated at a little over half of a billion, so that at least is a reasonable alternative, though I still want to point out that that mission is much earlier in the planning stages, and missions that diverge a lot from previous missions are more likely to have ballooning costs as new found kinks are worked out.
Another issue is that not only are the Mars missions promising more, but there is a much greater chance that they will be able to live up to those promises. Every single Mars mission we've done so far has added to our body of knowledge on the planet, and our ability to better plan a mission and engineer a craft that can get more and better data on the next run. From Viking and on we have answered many, many questions about Mars, and learned about even more questions (meaning that we know the sort of doodad that needs to be on the next mission to answer that new question). Starting a new series of missions to a new celestial body means that in a lot of ways you have to start back at the drawing board again. This is another reason to start small on a new body, better to have 3-4 partially successful $200 million missions leading up to that big $2.5 billion dollar rover mission rather than trying plan a $2.5 billion mission right of the bat.
I should clarify that I don't think that investigating these moons is a bad idea. I think it's a wonderful one. However I don't think that we should investigate these moons in place of Mars, when we have already accumulated so much experience on how to investigate Mars. It's also worthwhile to note that this was the viewpoint of every scientist interviewed in the article. Nobody said that they didn't want to go to Mars, they all said that they wanted this moons visited in addition to Mars, not instead of.