Not as bad as I thought. Water counts for this which makes it far easier.
Find a place with a large amount of metal and water. Just extractor the water and build water tanks. Whenthe tank is full destroy it then remake it. I used most of my funds to drop in machine parts to keep the extractors working.
To speed it up a bit you can take the starting class that gives fueled extractors and take a fuel refinery prefab with you.