The $250 charge indicates to me that delta does not actually want to offer this to lots of people. They want to make it available for those that really want it and will pay for it. For someone who is flying on a $8-10k ticket, probably someone else did the booking and spent someone else's money and an extra 250 in a very expensive trip won't matter.
Delta seems to not be looking to make money here, they are looking to not have to do this for many people a day but to have the option for certain HV pax who will then boom expensive D1 tix.