Update 27. 05. 2018: This weapon is no longer available in the game. I will update the solution if another chance to get a cheap Eye of Reach pops up in the game.
There's a really cheap Eye of Reach patched in today ( 01. 05. 2018 ) that only costs you 1 gold.
Just go to the weapon vendor on any outpost. The Eye of Reach for 1 Gold is the first item on the list:
Screenshot from the patch notes:
This gun is only available for the next two weeks. Grab it while it's hot.