Good question!
A Steem account costs 3 Steem to make regardless - this is a fee that is imposed by the blockchain itself. When you sign up for a "free" account through https://signup.steemit.com/ then SteemIt is actually paying this fee for you in exchange for some personal information (such as, I believe, your phone number and email address). It also takes a week or two to process.
Purchasing an account through our service, however, is instantaneous and very fairly priced - we pass along the fee of 3 Steem to the customer and charge an additional 1 Steem for ourselves.
RE: How to purchase a Steem account through BlockTrades