Get 250 Amazon Coins on your first Purchase of select Apps
edit: adding details
This promo works by giving 30 coins per $0.99 of the price plus 250 bonus coins for the first purchase of one of the qualifying apps (2,188 apps). Like you said, sorting by price will reveal the free apps (6 of them). Since they're free, they don't earn coins; but will earn 250 bonus coins when one of them is purchased. No matter how many of the 2,000+ apps you purchase, you only get the 250 bonus coins once. After that, you just earn the coins being offered for the individual apps -- 30 coins for a $0.99 app / 60 coins for a $1.99 app / 90 coins for a $2.99 app / 120 coins for a $3.99 app, etc.
Note: promo coins expire in a year, purchased coins never expire.