You aren't going to get much for $50 or less. The only real option is this:http://www.newegg.com/Product/Product.aspx?Item=N82E16822236067
It's a 320GB drive, so you gain in capacity. This drive is going to be faster than your 40GB, but it is an economy drive, so it isn't designed for performance as much as it is efficiency, so the difference may not be too substantial. I would steer clear of all refurbished drives, in my experience they just aren't as reliable.
personally, I would save up a little more and get the previous one I linked, but if that isn't an option, this drive will work. Alternatively, if the 40GB is currently working for you then there isn't too much reason to upgrade unless it is full.