Or 3 million people donating $2000 each for huge discounts on long term access. I would donate $10,000 if the discount was steep enough on long term use and included use of future expansions of the system.
Or better yet, sell tickets at a discount that can then be re-sold when the system is complete. Someone could buy $1M of reduced price tickets and then resell them for $2M when the system is ready. This way wealthy individuals wouldn't hit a maximum investable amount based on their personal expected use of the system.