I think evidence that themes are priced lower than they ought to be is that theme shops are turning to plugin development. A user might pay a certain amount for a theme then and even greater amount for a plugin. The theme involves every area of the site. The plugin involves one area of the site. That plugin's price is fine. The theme's price is wrong.

The only reason people feel like $100 is too much for a theme is because most themes cost $40 - $60. That's what they're used to. $100 would seem downright reasonable if the norm was $100+

It's clear that prices need to go up. The problem is somebody's got to be first. If ThemeForest was first, it'd make things really easy for independent shops. That's not going to happen though. Independent theme sellers will have to get a little uncomfortable and see what happens.

My personal thought is incremental price increases (ie. from $50 to $75 then possibly $75 to $99 at a later time) and no increase on renewal for current customers. That way the waters can be tested halfway and people won't freak out because the cost doubled overnight. The result will hopefully be the same revenue with less support expense. More breathing room for the seller and more resources to provide a better product and service to the customer.

I don't see $500 themes ever being a thing, not unless that includes some other value (ie. doing things for the customer). Maybe $50 - $200 with $50 being simple, low support blog themes and $200 themes being more sophisticated and hyper-niche themes from very reputable providers.