NEED ADVICE ON BUYING A CONVERTIBLE!!

We're going road-tripping for two months next summer and want to buy a used convertible (Make/Model doesn't matter). Could anyone please advice me on how much I would have to be prepared on spending??!! (Are we talking §300 or §3000???) Thanks for a great site and thanks to all of you Americans for your hospitality and for being the most friendly people I have ever met!
See you next summer!!
(By the way... yesterday I joined with 57.000 more Swedes to see Mr. Bruce Himself perform in Gothenburg. We'll keep that guy here...)