>Yet when people are selling the same thing for $4, it also looks like a lot, even though it isn't.
Here's your problem: you assume it's "the same thing". Just because the $4 thing looks OK doesn't mean it has the same code quality underneath, or that it wasn't stolen (as lots of "premium themes" are).
I'm sure many of us have spent and hour or two browsing theme, finally buying one, find out it's not going to work, and then start the process over again.
I've bought themes, both Bootstrap and others, that looked great, were dead cheap, and never got past the first open of the templates in Sublime Text. They were just too much of a disaster to work with.
Knowing that I'm getting top-notch quality is worth the $99 for me, if the themes fit what I'm trying to do. The amount of time saved not undoing someone else's mess is worth much more than $99.
That's not to say a cheaper theme isn't going to be good. I've also bought many that were quite good. I guess the difference here is that you can really trust what you're getting, and not have to rely on user reviews. I believe the Bootstrap people have earned enough trust where I know I'm getting a theme I can easily work with.
Here's your problem: you assume it's "the same thing". Just because the $4 thing looks OK doesn't mean it has the same code quality underneath, or that it wasn't stolen (as lots of "premium themes" are).