So I know this has been discussed before, notably a few ago based upon the search I did. Anyway, I know a lot of people use free WSO's as methods to build a seed list or build hype in a product they are about to launch.
However, I have been questioning the return on a free WSO versus an extremely cheap (say $1) WSO.
I think there is a distinct difference between a "freebie hunter" and someone who is willing to spend some money on information.
I think you can argue that by using a free WSO to build a list, you'll end up with people on your list that have absolutely zero intention of purchasing from you. Regardless of how good your future product is, they'll never buy it.
Whereas if you charge just a dollar, although you may end up with a smaller amount of "purchases", the quality of your list will be greater because those users are ready and willing to pay (even a small fee) for information they'd like. And as long as you give them a good product (although small), it more than justifies the $1 cost.
Just surfing the WSO section I haven't noticed any $1 (or really cheap WSO's) visible on the first 5 pages, however, there seems to be 5 or 6 free WSO's per page.
Hypothesis: Although you'd be charging for a WSO you'd normally give away for free (for the purpose of list building/ create hype for a future launch), you have created a higher quality list, versus a higher quantity.