[UPDATE (Monday, May 2): There have been a lot of interesting comments on this post since I put it up, but I have to give the honors to this one. Thanks, Jill!]
John at μcosmologist has created an interesting infographic depicting how much it would cost to run SETI from one year ($2.5 million) versus various other things we spend money on. In the graphic, each radio dish represents $2.5 million. Here's a (small) piece of it:
[Click to enalienate.]
The whole thing is much larger, and you really need to see it. Especially the bit about how much people spend on Starbucks. Yegads.
John made this because of SETI having to mothball the Allen Telescope Array, and I strongly suspect because people were trying to say there are better things to spend money on. I'll tell you, I think that argument is a crock. First off, it's a false dichotomy; we can afford to do more than what we need to survive. And moreover, there is always something better to spend money on, yet we still seem to be able to justify (or rationalize) the way we spend the money we do.
In the United States alone we spend five times as much on tobacco products as we do on the entirety of NASA. How's that for rationalization? And what we spend on NASA is much, much more than we spend on SETI (7500x more, actually). And we don't spend enough on NASA, either.
On the other hand, as a skeptic, I understand the desire to ask why we should spend this money on SETI, why we should spend it looking for alien intelligences? There are lots of reasons, actually, but I still think the best one is simple: Because we should.