Buying SAS as desktop drive is simply stupid, as even 15k rpm disks can be slower than 10k velociraptors, due to firmware optimizations. SAS disks are optimized for high queue depth random I/O; while desktop systems are very low queue depth.
But i thought we were talking about "RAID edition" or "Enterprise edition" disks versus normal disks; for example the WD Green disk also has a RAID Edition "RE4-GP" series drive, which is more expensive.
If a HDD manufacturer releases both a normal and an "enterprise" edition of the same drive, which share the same manufacturing process, there is no real difference between them. The difference is on paper: you get more warranty, some firmware options are enabled by default (TLER) and some specs are different. But physically it should be the same drive; so i wouldn't pay a premium for that.
On the other hand, if you can get 2 years additional warranty for just $10-$15 extra i would do it, but certainly no $50.