I have purchased about 60GH/s on CEX.io, and in my search for other sites that don't bend you over and screw you with maintenance costs, I found other sites like GAWminers.  For a few dollars on eBay, I can buy 1-2GH/s of lifetime CEX.io mining, wherease on GAWminers and some of the others, you are looking at numbers such as $40 for 1MH/s.

At face value it looks like you can buy 1000x the mining capacity for a fraction of the price, but I am assuming the situation is more complicated than that.

How do we explain the discrepancy in cost per unit of mining rate?