Don't forget you earn elite qualifying credit when you redeem chase points for travel so qualifying criteria may skew things one way or another.
It all depends on the day and route. I just booked a trip where 1.5 cents applied to the cash price made sense. How many chase points did I need to buy the cash ticket, minus how many UA points I'd earn. I could convert those points 1 for 1 to UA points, and compare the net chase cost to the UA cost. This was for a transom flight with lots of competitors and low flights.
I had a another example, roughly 200 mile regional flight on United Express with no competition and high cash prices. Flight wasnt full and mileage redemptions with UA miles were a great value. This is just like your delta dca-rdu example.
Overall, it depends, but hopefully some examples of my recent redemptions help you create your own value framework.