Upvote:4
This is really a good question, and It is suprising to verify that this is something that already exists on the web!
I'm talking about the Kicktraveler platform, "where traveling meets crowdfunding".
As stated in the help section of the website
Kickraveler is a tavel-crowdfunding platform. People who want to take a trip and that are looking for other mates to go with, can tell the world their travel program and wait for others to join in. Kickraveler is also a place to discover trips. People who are looking to go to any destination can browse Kicktraveler, pick a trip and join in.
Without copying the whole help section, I just redirect you to the "how it works" page, then you can easily browse from there.
It seems to be exactly what you are looking for.
This is a project which is in its early phase, so that it does not host many travels, but on the other hand it has the enormous advantage that the platform doesn't charge you with any additional cost.