Some on this forum speculated that the only real difference between 3.5 in. drives as Enterprise and as consumer drives is the warranty, and marketing approach.
I haven't been on the manufacturer side in some time, so I can't say for sure. However, I've been told that the RMA process (replacement drive and shipping) results in a net loss on the original drive, even if the defective drive is eventually repaired and used as a warranty replacement for some other customer.
Plus, the qualification process can often involve sending drives back and forth to the manufacturer. I had 100 enterprise-grade drives that went back to the manufacturer a couple times when qualifying them for my product. I'd assume that Dell, HP, etc. use a higher quantity than that when qualifying a new model, but of course they buy a lot more than I do, so the cost is spread out over a larger number of drives.
When negotiating an OEM purchase of enterprise drives, I've found that the biggest effect on price comes from selecting a different warranty - moreso than the quantity of drives specified in the contract (for, say, a $1M contract). Warranties range anywhere from 30 day DOA replacement and then I'm on my own, to the full 3 or 5 years that the retail version would have. Other things that effect the price are the batching of returns and the required turnaround - for example, returns of multiples of 200 drives with a 60-day turnaround is a lot cheaper than returns of 20's with 1-week turnaround. There's also a penalty for returning drives that test as "no problem found" if it exceeds a certain percentage specified in the contract.
This makes me think that the warranty expenses are what cuts most into manufacturer profit.