A person could conceivably start a small business making jewelry, keychains, etc.
Entry level CNC machines are around $100.
Not sure about viability, but that could be something worth looking into.
You don't have to settle for a business that small if you're making 40$ a day. In just a month, you'll have $1200 and you can have much better investment with that. 3D printer is popular in first world countries because they are the first ones to have and afford it. I think it would be good if you buy a 3D printer and then rent it or just accept the design online, print it, and then ship it to the buyer. Not everyone can afford a 3D printer but needs to use one.
But again, with 40$ a day, there would be much better investments than that. Research what your area actually needs. You don't want to buy a 3D printer and have it rented by other people, but end up selling it the next month because you just found out that most people in your place either don't need one or already one.
Well if you can make awesome things with that 3d printer then you invest your money there. If you can sell things made from it, then that would be really great. However this is really hard to do as 3d printers are made for small project and not really for purposes of mass producing goods.