A replacement LCD would most likely cost quite a bit depending on the model. I could actually cost more than the TV if special ordered from the manufacturer. That's because typically TVs are loss leaders in big box stores and tend to sell at or below cost. That cost is also reduced due to large volume orders and special deals between the manufacturer and the retailer.
Decent 42" LCD's can be had for under $1000 these days and more price drops are around the corner during the Xmas season.
A viable option is to search around for a used or damaged LCD with the same model LCD panel. Often times the ballast or back-light will go and people just ditch and buy new thinking the TV is dead. Salvage the parts from the one to repair the other. It can be pretty hard to find a replacement that way however and you have to be diligent.
You could try Ebay, kijiji.com, a local electronics recycler, a local TV shop or contact the manufacturer.