We only had 3 days in the DC area to see all we could see. Looking back, I wish we hadn't spent the time and money to go to Mount Vernon. Don't get me wrong. It was beautiful, educational, with plenty to do and see. We had budgeted the entire morning to this one attraction. (30-45 drive followed by 2-2.5 hrs. sightseeing and then eat in the car on our way back to DC) We ended up spending more time than planned, which meant we had to sacrifice something else on our super long list on DC attractions. And, pay a hefty sum to eat at the food court on site.
Since there is so many free or low cost things to do, I think this should be left for a longer trip or maybe the second time visiting. I would suggest travelers give a full 3-5 hours at Mount Vernon if you are going to pay the ticket entry.