50 pin SCSI drives are getting more expensive, 68/80 pin (U40/80) are now getting dumped cheaply, and 68/80 pin (U160/320) are still in use, large capacity, and expensive.
Most if not all of the server drives are 80 pin SCSI and you need the correct adapter to use them on an older Mac (at reduced speed if the drive will sync down low enough).