A $20k pimped out setup or two is still less than a recruiter would charge, and hopefully about the same as the value of a developer's work to the company in 1 week.

I think Asana has the right idea, just letting people pick whatever they want within a $10k or so budget. There is some value in having commonality (spare parts, support, chargers), and I'd be surprised if anyone wanted anything except Apple or Lenovo laptops and high-end desktops with big monitors.