Keep in mind that just because you see a car listed for a certain price doesn't mean that is what they are selling for. Normally CO's seem to list at a higher price because owners like to think that because there were fewer produced in this color then it is rare and therefor more valuable. Truth is that Ford made less of the color because there is less of a market for it. CO is an awesome color (especially with chrome rims!), but there are less people out there who want an Orange car. Balancing that out is the fact that there are people who hold out for CO and when one pops up at an affordable price it might get snatched up quickly as a result.
At the end of the day, list it at $18.5k and see what happens.
Whatever you end up doing, good luck . And hey ... if you can't get what you want for the Cobra you could always just keep it (it is your favorite car!) and get rid of some of those GM's you have sitting around :beer: