Jet fuel costs - up more than 80 percent over last year - are forcing low-fare airlines to sharply raise some fares, and reinvent themselves to appeal to not just bargain hunters, but also business travelers, the New York Times reported today. Airlines like Southwest, JetBlue and AirTran have been able to offer cheap fares for years because of their lower operating costs, for reasons that include simpler jet fleets, work rules and less-sprawling route networks. Their low prices and rapid growth forced the largest carriers to cut fares whenever they entered a market. They still offer deals for passengers who book trips well in advance, travel off-season and at less popular times. However, bargains are getting harder to find, as low-fare carriers join the bigger airlines in raising fares, which are up about 18 percent industrywide this year.