Chillingo, the game's publisher on iPhone, has also taken the increasingly popular approach of releasing COGS for 99¢ and then offering additional puzzle packs that, if purchased via the App Store or in-game, bring its total price up to about five bucks. From what we've played so far, we'd say the initial investment is more than worth it -- we haven't downloaded any additional content as we're still getting through the included puzzles. (It's tough!) Find out more at the handy-dandy App Store link below.
COGS (Chillingo / Lazy 8 Studios, 99¢):