Your price is out by $100, since the $30 upgrade license for Snow Leopard is from Leopard only. The upgrade from Tiger to Snow Leopard is $129 (i.e. single Snow Leopard license in OSX box). Sure, you can use a Snow Leopard upgrade directly on Tiger, but you're breaching the license agreement if you do so, and doing the legal equivalent of pirating Leopard.
The price issue is anyway besides the point, given the different feature sets of the OS's and their hardware requirements. I was simply pointing out the difference in the apparent price of the upgrade versus the actual price if you aren't on Snow Leopard, and balancing that against the benefits of the upgrade route, i.e. you can't get screwed by upgrading just before another OS release.