Ranchos are the cheapest, but the initial reviews I would wait and get some more long-term reviews. Sway-a-Ways or Fox are going to be your cheapest that already have a good reputations behind them. Give PolyPerformance(.com) a call, they're very helpful. Also, airshocks would be cheaper than coilovers, and have their own set of pros and cons.
more importantly, why do you want coilover? there are a lot of guys here that have really pushed the performance of their XJs, and very few have found coilovers to be necessary for that.