Re: Windows 8.1: 'It's good for enterprises, too,' says Redmond
You are correct in that the only volume license SKU is for a Windows Upgrade - this has been the case since Windows Vista I believe. We still have to buy an OEM license with the hardware, but as you originally posted, this is not normally an extravagant cost. My sums still show the OEM+upgrade license being cheaper than a retail copy, and you get the option of adding software assurance to get the Enterprise functionality that you want (BitLocker), as well as custom imaging rights, one VL key common across systems, spread of payments across multiple years and delaying license purchases (depending on which agreement you choose).
I'd better stop now, as I'm not a great MS fan, but there are ways of getting a workable solution in place where you have to :)