Persons sells some cheaper leather saddles, but their quality is apparently quite poor. Rivendell and Velo Orange have their own models, but they're just as expensive as a Brooks. Your best bet is to keep an eye on Chain Reaction Cycles. They occasionally have some pretty steep discounts.
Also, remember that a Brooks will last just about forever. A synthetic saddle that costs half as much but wears out twice as fast isn't really any cheaper.