I've been trying to learn about marketing and found an artilce titled "Zero as a special price: the true value of free products" .
The article is written by 3 college professors after studying the pricing subject. It appears to relate to IM very nicely since, we come in contact with free products every day. The biggest thing that got me was when compared to different prices, the reaction was different. Although the math is a little difficult, the description of the results explains the everything quite clearly.
If you find this interesting please take the time to contact me, via IM or PM. I have read some other thing that will give you a different perspective on pricing models.
Looking forward to hearing from you.