i think your budget is too small for any decent 50" television.
if you are willing to downgrade to a 40" then you can likely find some good products from sony, samsung or lg in that price range when they go on sale.
even a brand new vizio is above your price range unless it is on a very good sale and that is a cheap brand of television
are you looking for the absolute cheapest 50" television you can get or do you want something half decent because the two terms really arent interchangeable.
just my thoughts...