I would try to get them even lower on price, mention that it is a lot of miles for a 3 year old car. Also, a little FYI, all convertibles are factory upgraded to the premium package. what you think is an option is most likely standard equipment.
Mine only has two options and it stickered for 48,100. take off the performance pack which you couldn't get on a vert in 15 and you're still at 45K for a basic gt convertible.
Point is even a stripped vert is not cheap, but it has a lot of stuff and standard equipment. Only the base v6 vert came as a standard mustang and could be had in the 20's new. So on second thought, maybe 24K is a good price, but I would still try to get it lower.