You can find a 55 kit (tank, hood, filter, heater) at Walmart for around $180 although the filters, etc aren't the best quality. Pet-smart in my area had 55 tank+lid for about $150. If you have a Big Al's in town, check them out, last I look, the 55 tanks are about $130 or so. They do have a "package deal" where you get pretty decent equipment (Emporor biowheel, good heater, etc) with a 55 tank for around $250. That is the best deal I've found considering the quality of equipment.
I did however, managed to find a much better deal used at the local classified, although I was looking for over 6 months. Another place to look for is ebay. People won't ship tanks that size, but if someone in your area is selling, you might find something for a good price.