According to the listings over at Geizhals.at, the cheapest HD 4850 that you can currently find is Powercolor's card listed at €134,71.

The card isn't yet available at this price, but it is available at a different retailer/e-tailer for €136,26, which still makes it the cheapest card around. It works at reference clocks and just like most of the available HD 4850 cards around, it is cooled by a reference cooler.

You can find it listed here.