Sorry, A Skylark

Buick convertible what? You don't say what type of model it is which makes it hard to determine a price. Or perhaps you mean a Buick Wildcat? If so, I don't believe a 355 Wildcat engine was an option. And by popular do you mean how many were made? Price is relative, it all depends on condition, location, buyer, options, originality, etc. It could be anywhere from $1000 to $30,000.