Yes it is completely free. THIS is the ONLY website you should ever go to for trustworthy jailbreak information. http://blog.iphone-dev.org
BTW, technically it does void your warranty, BUT it is completely legal, and if it unbootable then Apple techs can't tell it is jailbroken, so they must honor the warranty, and if it is bootable you can restore it first if you ever need warranty work done. Restoring removes all traces that it was jailbroken and restores the warranty.