Do you plan to continue to release videos and grow your channel? If so, then there's a good chance they'll end up taking more than your signing bonus.
If you plan on quitting soon, then feel free to take the money and run.
Unless you have (in writing) that they're going to build up your channel or that you think the perks they give (music, dashboard, etc) are worth it, there's very little reason to join a network.
If you continue to grow, it may make more sense to find someone to represent you and assist in gaining licensing deals and so on - which a network may do, but (again) only if in writing.
80/20 split for your size seems a tad high. You should be able to get closer to 90/10.