I recently purchased a used 2010 HP Z600 Workstation on eBay for less than 300 Euros. It’s a “Mercedes of computers,” solid as rock and was very expensive when it was new.
The Workstation came with 2 Xeon 4-core E5630 80w processors, 8GB of RAM, a Quadro 4000 graphic card (still costing over $100 on eBay), and a Windows 7 sticker.
I discarded the existing 8GB of memory (2GB modules) and added 6 new 4GB DDR3 ECC server modules for just $50. Now, I have 24GB of RAM for only 50 dollars (the machine limit is 48GB) as server memory modules are very cheap.
With 2 2.53GHz Xeons (2.8GHz turbo), this system is faster than current top i7 processors and costs less than a comparable processor. In the 7zip benchmark, I get 30,000 MIPS while an i7-7700K gets the same score. However, it is worth noting that this dual-processor system consumes a significant amount of energy. The processors alone draw up to 160 watts :)
For just $100, I could buy two more powerful 6-core Xeon X5670 processors with 24 threads, which would almost double the performance.
A really cool addition is that this computer has a built-in FireWire port which I use along with Presonus and M-Audio interfaces.
2020 update: I’ve changed processors to 2x 80W E5649, (2,53 GHz with 2,93 GHz turbo) so now i have 12 cores and 24 threads :)
Old Xeon Workstations are cheap and powerful. ECC memory modules are cheap as well :)
This computer accepts and recognizes old PCI 5V sound cards like the Motu 424 or even SB Live or Yamaha YMF724 with a hardware OPL-3 chip. You can use it for old FM trackers or dos games with real opl-3 sound :)
some reviews from the past:
https://www.cgchannel.com/2009/10/hp-z600-workstation-review/
https://www.geek.com/chips/review-hp-z600-workstation-844631/