As far as I understand it in retrospect (having grown up in a strictly PC household and later investigating "retro" platforms where SCSI was standard), SCSI was not inherently expensive. It's more that for PC platforms, ATA provided a hack that was cheaper at the time, which led to a bifucation of the market. ATA roughly turned HDDs into a subset of the ISA bus, which in turn originated as a buffered version of the CPU bus. That led to SCSI being at a premium on PC platforms because the target market was limited to customers for whom ATA wasn't good enough (mainly because they wanted more drives or longer cables than the thin translations of ATA could support).