I don't have a clear answer for you. I'm using the oneplus one I got for $300 CAD used; the 64gb version.
It depends... I don't have a phone contract right now and the company I'm with is offering me a new phone with a plan. I can't think of any phone I would replace it with factoring in the cost. I know you're not asking about other phones, but I guess the point I'm trying to make is if the price is right and you want something that works, the oneplus one will still do that for you.
If you value the next thing (even with no big changes in specs), get the next best thing if you want to dish out the money.
