I have been living in the past, and it is finally catching up to me.
What is the cheapest way to upgrade from Mac OS X 10.4 to 10.6?
Spend the 30 bucks to get the 10.6 DVD and then use it for a clean install.
There's no reason to buy 10.5 so you can "upgrade" to 10.6. The 10.6 DVD is exactly the same as all the other release DVDs.
Well, the cheapest way to upgrade to Snow Leopard is to find someone who has a legal DVD but is no longer using it, and ask him/her to give it to you.
(Note: IANAL and I don't know if Apple's EULA "allows" such transfers, but you did ask for the cheapest way, and I'm assuming that you do not want to simply use Bittorrent.)
If it was me, I would definitely spend the $30 to buy it from Apple.