I had a "Rabbit" fastloader cartridge way back, and it was a fastloader for cassette drives! Kind of a hardware version of the old Turbotape.
As far as value is concerned, other than being a collectable, it probably wouldn't have much use. That was back in the day when 1541 drives were 800 dollars CDN... I know because my folks paid that for my drive. Was a great alternative to the "super fast" 1541 device!
Nowadays, five bucks will get you a 1541 drive... Not many people will make tape their device of choice.