Blockchain itself IS a best proof possible. Actually, this is its main purpose - to be a very reliable proof of various important information.
And links to actual contracts in the blockchain right above.
It just example though - you can pick up any of your own contracts and see the same. I have checked few already.
And there are no any contradictions with what the Nebulous wrote. They said what they are getting 3.9% of total contract funds via siafund.
I am saying the same(but with more details) - they collects 3.9% of total contract sum, but these expenses shifted to the renter side. Host pays 0% fee to sea-fund and renter pays about 10% or more fee - depends on ratio between host prices and host collateral. Bigger collateral = bigger fees from renter to seafund.
This is the reason why setting collateral too high is a bad idea despite the fact that this increases the score and trust of the host. Simultaneously, this significantly increases the renter's costs and reduces the attractiveness of your host.