I have a free version of my car materials, I dont think its negatively effected sales of the full version, but that is mainly due to the type of product.  In the free version you get around 30 materials, but the full version has 167, greatly expanding the free categories with more options.  Here the free version I think attracts sales, as people want more choices once they get a taste of it.

 

That is not really possible with figures and objects.  Unless you released a free non UV mapped version. I am sure there would still be people out there who are satisfied with even just a basic non-texturable model. They do this over at Turbosquid. Well a few sellers do. It seems to work for them, and I can see in principal why.  For a figure you would get to look at the model in your own app and lighting/style but unable to texture it, or pose it (without a rediculous amount of work). a very low res non-uv but posable version could work too I think.
Also generally you would not see a quick gain in sales, as purchases are delayed by having the free version to play with.  How stripped the freebie is will decide how long it takes before people decide to "upgrade" to the full.

 

with a free version that is geared towards bringing more purchases you need to way up the amount of lost purchases against the amount of additional purchases the exposure the free item brings you.  Some people will be satisfied with the free version and you lose a purchase, whereas some people who would not have purchased before will be convinced to buy thanks to trying the free item.
On top of that, in order for a free item to make any real difference at all, it needs to reach a wide audience, which can be just as hard as making a purchasable product reach a wide audience. You would need ALOT of downloads to see any noticable increase in sales past daily fluxuations.

If you are an established seller with a large client base of course, this is all extremely relevant, but smaller sellers will mostly just lose out on sales releasing a free version.

TemplarGFX
3D Hobbyist since 1996
I use poser native units

167 Car Materials for Poser