I don't think there is. 10.5 would be cheaper. Your only hope would be if your Mac shipped with 10.4, you could buy the system discs from Apple fairly cheap.
There are several vendors that still have legal retail Tiger packages available for sale as well as some of Amazon marketplace sellers (some of which have used copies too). The key is that because of the demand for those packages and the dwindling supply they are not by any stretch of the imagination cheap.