Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
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. :eek:

Amazon 1
Amazon 2
Amazon 3
Hard Core Mac
We Love Macs

Some other sources
 
As an Amazon Associate, MacRumors earns a commission from qualifying purchases made through links in this post.
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.