I'm pretty sure Dell and HP provide support on all their Ubuntu machines, as do System76 et al. And they have extended support offerings all the same. Not sure on pricing, though.
Software support, not hardware. Patches, etc. RedHat is pretty much the only distro maker that will provide software support for the same timeframe as Windows releases are supported. Ubuntu, you'd have gone through the compatibility testing, upgrade testing, migration, etc cycle at least once more often than Windows, and deployments for anyone with custom software get expensive fast.
Besides RHEL there is also SuSE SLES / SLED with apparently 7 years of "general support" + 3 more of "self support" (guess that means you just get serious security fixes).
In addition to the shorter support time, with Ubuntu LTS it's a bit too easy to install packages from "universe" that aren't actually supported for more than 18 months...
