That was easy. I've done this a couple of times before and at this point building one takes me under 20 minutes.
So the breakdown is this.
$2 Solar Panels
$1 Battery holder
$0.02 Blocking Diode
Time: 20 minutes.
You can use this EXACT same setup to power little light up projects. Throw in a few transistors and resistors and you can make a dark detecting circuit for all of $0.20 more.
If you're looking for solar panels or little solar kits I have several available on my website browndoggadgets.com.