I have no idea how much a solar panel cost so after a quick search, I found this link:
http://www.solarhome.org/21-30wattsolarpanels.aspx
They're listing a 21-30 watt SP for over $200 so that might explain why the solar powered attic fan is so expensive. I'm wonder if a panel can be bought cheaper........there isn't much ROI if you have to pay $200+ for a 20w to 30w panel.