DuPont Circle area condos sell for anywhere from $300-$1,000 per square foot. It depends on many factors, condition, location, view, amenities such as a pool, doorman, front desk staff.
The average is around $500 per foot for a nice one bedroom that is about 500-600 square feet.
The smaller the unit the higher the price per foot, because it has all of the expensive stuff, kitchen, bathroom, HVAC. Living space does not cost as much per foot as does your kitchen and bath.
I hope that answers your question. Inventory is very tight right now so use a savvy agent who really knows the market.
I know and work the DC market and am a fourth generation Washingtonian. I can be reached at firstname.lastname@example.org