Vim not using variable value when changing a setting

Question

I have the following vimscript:

let g:base_path="~/.config/nvim/"
let g:plugins_path=base_path."plugins/"
let g:repos_path=plugins_path."repos/github.com/"

set runtimepath+=g:repos_path."Shougo/dein.vim"

But it's doing something weird, instead of seeing a path at the end, I'm seeing this:

runtimepath=~/.config/nvim,/etc/xdg/nvim,~/.local/share/nvim/site,/usr/local/share/nvim/site,/usr/share/nvim/site,/usr/local/Cellar/neovim/0.2.2_1/share/nvi
    m/runtime,/usr/share/nvim/site/after,/usr/local/share/nvim/site/after,~/.local/share/nvim/site/after,/etc/xdg/nvim/after,~/.config/nvim/after,g:repos_path.

It's adding this to the end:

,g:repos_path.

instead of

,~/.config/nvim/plugins/repos/github.com/Shougo/dein.vim

Why is vim not adding the path to the end correctly and also why is this happening?

EDIT: The only way to get this to work is to use the fullpath without any quotes like this:

set runtimepath+=~/.config/nvim/plugins/repos/github.com/Shougo/dein.vim

Answer 1

After reading this:

let &backupdir=s:vimetc.'backups/'

I realised what I needed to do was this:

let &runtimepath=&runtimepath.','.g:repos_path."Shougo/dein.vim"

I have to say, vim script can be a bit inconsistent, using bare strings works fine but without it you need to some some weird stuff to get it to work if you want to use variables.