You should actually probably stop referring to the Windows 8.1 Enterprise Evaluation version as RTM. While technically it has been Released To Manufacturers it is really a 90 day Evaluation (Trial) version and is time bombed to expire.
Just so people don't misunderstand and think that they are getting something that they are not.
I though it was RTM. Well it's still only a temporary build that will expire 90 days after you install it. So yes it is only a trial version same as Windows 8.1 Preview was. Though it gives you a chance to test Windows 8.1, before you decide if or not you want to buy it.
Fortunately there are different subscription levels. I can assure you I didn't pay £600 for my Technet subscription, my last renewal was £93, about what it would cost to buy Win 8.1 Pro at retail. As an example of what's included at different subscription levels, my subscription includes Win 8.1 & Win 8.1 Pro but not Win 8.1 Enterprise.
I have a Technet subscription for my personal use and have a MSDN subscription via my employer. I have no idea what my employer pays for my MSDN subsciption but I need access to all MS operating systems and applications in my job as a developer so my employer provides that to me.
Yes Microsoft have 2 website with subscriptions-Technet and MDSN. The MSDN one was £600 a year it said.Which entitles you to free OS downloads from Microsoft including Windows Vista Business with Service Pack 2.
The Technet one was just as expensive and I cannot afford either. So that's out for me. As for the free update to Windows 8.1 I cannot even get that, because you have to go though the Apps Store. As you know my netbook won't run Metro Apps. Also the high street store in England are not selling Windows 8.1.
So the only way to get it here is to buy online from the Microsoft website. Then I think you can download the ISO. Or you can order from Microsoft over the telephone and they will post the DVD to you. This was not the case with Windows 7 or Windows 8, as you could buy it from the shops,without going online.