There's an interesting chapter on behaviours associated with 'Free' in this book:
"Dan Ariely: Predictably Irrational"
Which does cite actual research.
There's a wikipedia synopsis:
In chapter 3, Ariely explains how humans react to the words "free" and "zero". Humans make decisions without rationalizing the outcomes of their choices. To illustrate this point, Ariely conducted multiple experiments. The outcome was consistent: when faced with multiple choices, the free option was commonly chosen. With the opportunity to receive something for free, the actual value of the product or service is no longer considered. Ariely claims, “Most transactions have an upside and a downside, but when something is FREE! we forget the downside. FREE! gives us such an emotional charge that we perceive what is being offered as immensely more valuable than it really is.”
Ariely's concept of "FREE!" applies not only to monetary and quantitative costs, but also to time. We forgo some of our time when we wait in line for free popcorn or to enter a museum on a free-entrance day. We could have been doing something else at that time. Ultimately, he demonstrates how such a simple concept can be used to drive business and social policy. For example, to reduce health cost, companies could offer free regular checks. Employees would be more willing to get them at zero cost rather than paying some amount of money. Ariely recommends the consideration of the net benefits of the choices we make regarding both preference and money. Perhaps we would get the better deal and even save money if we did not react to free the way we do.