According to data released by the game studio, the average price paid was $2.03—almost exactly a tenth of the game's full retail value, which was $19.99 last time we checked.
As you can see in this chart showing the distribution of payments, a large number of users paid just one cent for the title, and the majority chose to pay less than $2. However, a small number of customers paid well over the usual asking price—some up to $50.
Between October 13 and October 19, 2D Boy's Ron Carmel says his company sold 57,000 copies of World of Goo but lost 13% of revenue to PayPal transaction fees. If my math is right, that means the sale generated just over $100,000 in revenue. Interestingly, sales of the game through Steam (at the full price) went up a whopping 40% compared to the week before the promotion.
The celebration is running until October 25, by the way, so you still have a chance to buy the game at your chosen price.