Lifetime Roon is a go - alternative to Roon Nucleus

The Nucleus is built using an Intel NUC in a fanless case. It is not that difficult to put this together yourself if you are somewhat comfortable with computers. If you have ever built your own PC, it will be easy. 

https://kb.roonlabs.com/ROCK:_Getting_Started
https://kb.roonlabs.com/ROCK_Install_Guide

Do a google search for Roon Rock Install and you will find a number of videos to walk you through the process.

Building your own will cost you less than half the price of purchasing the Nucleus.

When you build your NUC, you'll have to select an M.2 drive to hold the OS and database, and RAM. Make sure you visit Intel's compatibility website to find M.2 drives and RAM that has been validated on the particular NUC you are using.

You can often find non-validated parts that will work, but based on the many posts on Roon's forum by folks that have tried to save a little money, you are asking for problems down the road. 

So I can buy a nucleus + ssd from Roon all put together for about 1800, vs 400 for nuc plus another 100 for both ssd and ram as prescribed by Roon.

that is a huge mark up , what am I missing.

That's what I wondered when I went this route. I spent a bit more getting a better CPU. If you're comfortable assembling, plugging in a M.2 drive and memory and installing the OS it's easy. 

You are missing a pretty and fanless case.  Some people are averse enough to DIY to pay the freight.  Others think they will get better sound.  Don't go overboard on the size of your M.2 drive thinking that you can store music there.  You cannot.