The investment requirement is "substantial" relative to the enterprise so there's no legal minimum but in my experience most consulates won't entertain E-2 visa applications where the investment isn't above $50,000. And understand that the investment has two components, the money transferred/invested into the U.S. company and the money spent by the U.S. company and a substantial amount needs to be spent and the percentage is higher the lower the investment. So, where the investment is as little as $50,000, most consulates will want to see nearly all of this spent. But to be clear, $50,000 is really low. And yes, the money can come from others as long as here the money is coming from Mexican citizens or Mexican-owned companies.