You may want to check out Amazon.com, it looks like they have a few used copies starting at $41.99. It may not be that big of a dip below $50 but it never hurts. Also, try GameFly.
I think Amazon had it for 39.99 awhile ago for their deal of the day. The game should drop to 29.99 once the greatest hits comes out (assuming it gets a greatest hits release like the 1st one).
Of course you can get a used one for under $50 dollars (e.g. $45.99 after shipping) on Ebay or through Amazon sellers.