Here's my take. You spend your spare time, which is valuable to you, working on this software you're giving away for free. You're expending your own resources - so why should money be any different? Its still a resource, right?
I guess it depends how much you love what you're doing. If you love it and you can justify spending the money on a download.com listing for some reason (feeling good about yourself, ego-feeding, one-upping your mortal enemy) then why not?
Think of it, say, as a car. You spend thousands of dollars on restoring, hm, say an old Mustang. Driving it every day wouldn't be cost effective. You'll still need to care for the thing. How much of your personal resources are you willing to spend maintaining and driving the car? Is it because you're trying to show off something or does it truely bring you happiness?