But when customers could pay what they wanted in the knowledge that half of that would go to charity, sales and profits went through the roof. Around 4.5% of the customers asked for a photo (up 9 times from the standard price plan), and on average, each one paid $5.33 for the privilege. Even after taking away the charitable donations, that still left Gneezy with a decent profit.
-- Discover Magazine 2010, July

When free / freemium just doesn't cut it, invoke blind reciprocation....