In terms of content, TTK will not have as much stuff as Destiny but it will be the same price. That's the simplest way to look at it. Plenty of people will be upset about it and that's justifiable. But there's different ways of looking at it.
On the other hand I would say if you averaged out time spent per game for a range of people and a range of full-priced games, you'd see something like ~20 hours as an average play time per game. The majority of people who buy TTK will get significantly more than that out of it.
Uncharted games rarely clock in at more than 10 hours and nobody questions their value. Yes they have a multiplayer component but I would think most people never touch it.
I'm not saying I agree with the price of The Taken King, Activision are just trying to make money where they can. On the other hand there's no monthly fee and no microtransactions. And at least they don't charge £15 for a fucking server transfer.