One of the original Spyders will go for something in the $300-400K region. The conversion, usally done by Starman out here in California, will go in the $150-250K area.
As the "Original" spyders were indeed conversions themselves built by Italian Coach Builder Scaglietti, commissioned by Ferrari. The coupes were delivered to Scaglietti (in shell form) and the roofs then cut off and chasis reinforced. A Euro version and US existed as usual. The most striking difference between the two it the US version recieved Pop-Up head lights and the Euro's sat behind clear Plexiglass. Also center knock-off Borani wire wheels were replaced later in the production run with center knock-off five star wheels that we all now associate with Ferrari cars.
During the 80s the TV show "Miami Vice" used a Replica of one of the spyders built on a Corvette chasis as one of the main (Don Johnson) character's car. Due to the exposure of the show and Ferraris unhappyness at a "Replica" being used they offered the show a White Testarossa as a replacement. Needless to say the show excepted. Don Johnson was also given a Testarossa as well. This show as well as "Magnum PI" helped move Ferrari from obscurity in the US to one of the most recognizable brand names in automobiles.