We need to allow a client to offer products for free (a price of $0.00) in the e-commerce module. We modified the code and posted the workaround in the Bugs forum:

http://www.kentico.com/DevNet/Forums.aspx?ForumID=7&Thread=00003905

But we would request that Kentico add a "Minimum Price" or "Allow FREE ($0.00) products" or some configuration option like that.