I've been looking for something like you, because i cannot afford to buy a nas at the moment.
What you are looking for is a JBOD system, and the only cheap one i can find is a Raidsonic with room for 2 disk's and the downside of that one is that i only has USB2. Nothing more, nothing less.
I cannot post a price, since I dont know where you live, but I live in denmark, and that is the cheapest solution I have found.