I'd say it is worth it if you can afford at least 200. I would say it is a good incestment because of how steady the price of atms are. They haven't gone up or down in the past several years. While you have them, you get a free 10 gems per atm every day. Then you can just re-sell them for the same price (or even a bit more if you are good at trading) that you bought them. So basically, it is better to buy 200 atms for free gems than just have 6 dls doing nothing in your inventory.