You can buy a new Dell without paying for Windows. Go to www.dell.com, under the Small & Medium Business pulldown at the top, select Desktops. Then on the sidebar, select FreeDOS. Not a barebones, but you won't pay for an OS. Not available on all models.

For used computers, I find it useful to check PacificGeek.com. They sell off-lease, repos, trade-ins, etc., all refurbed and checked out by PacificGeek. On any given day, they have plenty of low-price stuff. If you check frequently, you'll find some ultra-cheap computers.