An amount appropriate to the specific car.
I know that's a bit evasive, but there are so many factors involved in the value of a car (including things like shipping or the cost of a flight) that it's difficult for me to say.
Low to mid 20s is what I have in mind.
BTW - I'm only interested in convertibles not coupes. We have pretty decent springs, summers and falls here in Michigan and I want to enjoy some top down driving!