eBay is always good to see what's available and where prices are. SCSI drives are becoming less and less common, therefore you pay a premium for the drives (whether it's HD, CD(-RW), or DVD).
Baucom Computers has a Yamaha 4x for $43. It looks like a small time operation, because it is. I've dealt with him before and have no complaints; he's very accommodating and pleasant on the phone.