IMHO.... Paying >$100 for a plugin can be a good value, but not for a theme. I can use GravityForms on many different websites without having to modify it. That's not the case with most themes. Even some of the more complex themes and popular frameworks require modifications for something as simple as having a vertical logo (when the theme demo uses a horizontal logo).

The problem with themes isn't that the price is too low, it's that the model is broken. Most of us wouldn't design a website for a client, at a fixed cost, and then support it indefinitely at no additional cost. That's just stupid. I, too, would be pissed if I had to provide perpetual support to someone who paid $40 for my item years ago (especially if the 'marketplace' took a large chunk of my revenue in the first place)!

If anything, as a hosting provider I'd be willing to pay theme authors for things like:
-email notifications when a theme update contains a critical security fix
-proactive compatibility testing with each new WordPress version (as opposed to having to search through ThemeForest comments for anecdotal evidence of incompatibility/issues)
-diff showing changes in each release
-automatic updates