Dell Windows-free PC
You can get a Dell Dimension 5150n with no OS for about $649 US. But as The Register points out, it's not such a great deal.
Let me tell you a little story about our new house. It's being built, and we have the opportunity to make requests of the builder. For example, changing wiring around, moving walls, that sort of thing.
Every change, no matter how small, carries with it a management overhead. Because every deviation from the standard plan requires a change order, and requires that someone ensure that the change order is being followed, there's an extra charge for every change. Thus, simply removing a wall costs money.
I don't see why buying a PC would be any different. There isn't much demand for a system without an OS (contrary to what tech zealots would believe), so I expect what's happening is every time someone wants one of these systems, they pull one off the normal assembly line, wipe off the standard configuration, toss FreeDOS in the box, and then rebrand it as an OS-free PC.
Until there's significant demand for this sort of configuration, such that it becomes something that Dell is mass producing (and thus benefits from the economy of scale that that would enable), it's a special case for Dell, so it's going to cost extra.