A workstation is not a PC, though it may run Windows OS. If you are comparing prices you must consider the disk controller and/or array, robust oversized powersupply, optimum cooling system and other items. Does your managment run their web server, accounting software, etc. on a plain PC too...no they buy server hardware.
I have had one WS class machines (dual processors, RAID 0 hard disk array, professional graphics) which I used for six years...the only changes were graphics card and memory as the prices dropped on these items (and new OS, but thats for IT department reasons.) That dual P900 Xeon still runs rings around the P2.4's they bought last year (1MB L2 not 256kB, 2x 10,000rpm UW3 SCSI in Raid 0 disk not 7,200 rpm ATA.)
Spend 2000 every year for six years keeping your PC at top performance, or spent 6000 once in the same timeframe and maybe another 1000 after 5 years to boost it...thats how management needs to see it...its NOT a PC, its a WS.