Oh please. As I said earlier, the under the hood improvements are major for developers and power users.
I suspect that they're not charging as much since there aren't the tangible improvements that any consumer can get excited for (UI and workspace improvements, new included applications, etc). Nearly everything in 10.6 is under the hood: 64-bit apps from top to bottom, Grand Central to simplify programming applications for multicore CPUs, OpenCL for built-in GPU processing for regular applications, a HD footprint that is 6GB smaller due to filesystem compression, all of this on top of the improvements they've made to the included applications and a few UI improvements they've made (Dock Expose, etc).
If Windows was making so many radical under the hood improvements and changes, the absolute last thing I would call it is a "service pack".
I think both companies are entitled to charge whatever they decide to.
I'll be completely honest. As I don't use a mac, I don't keep up much on their updates. However as I stated Apple THEMSELVES at one point stated that it was just going to be a service pack more or less and nothing more. I was assuming that they were pretty much sticking to what they were planning from the start. Basically what they admitted at one point was that it was time for an new MacOS to be releases, they didn't have a new one ready, so they were just releasing a service pack (collection of bug fixes and a few new features) and calling it the next version. I guess they must have changed their minds then and actually decided to make it a little more than just a patch rollup.