ebay on an auction
machine came with dual X5550s, 24GB RAM, NV250 gfx, the HP coolers (as they are decent) which cost me £200-£300 (I forget the exact figure), it has the latest board revision that lets me use the 5600 series hexacore chips
Into it went - existing GFX from old PC (the 6770, cost £40 a few years back used), 48GB RAM (used) - £40 delivered, 2 matched X5670s - £120 from ebay used, 3TB HDD - £90 (less perhaps), 256GB Kingston SSD £50 at the time (the mrs acquired the Samsung SSD I'd bought for this)
I think I also put a cheapo USB3 card into it also (something like £10 or £20)
Whats next? New GFX card and maybe an NVMe SSD.....
(wife's machine is a Z620 - single quad core Xeon, came with 8GB RAM, upgraded to 32GB (again used memory from eBay) Samsung 256GB SSD, another 6770 GFX card (£20 or £30 from eBay) 2TB HDD think that again was £400-£550 all in)
They were popular CAD machines so there's usually a lot for sale on eBay along with Z400s, Z800s, Z420s, Z820's at varying price points