Not as straight forward as that Fin.
It's more about what you bring to the table in terms of appeal to the public in order to make the bout worthwhile to the promoter and what you risk losing.
For instance more people in this country (where the proposed bout will be staged and watched mainly by the British public) will have heard of Khan more so than Brook but Khan isn't actually a world champion at the moment but Brock is - so he is 'risking' something whilst Khan isn't and could simply 'milk' his title for a while by taking easier fights rather than face Khan.
So one is bringing a 'crowd' and the other is bringing a 'belt'.
A balance has to be reached by both sides whereby they think the purse they will get is worth the risk of losing - sometimes it is worth to them to take the smaller purse because if they won the fight (or even do well but get beat) against a 'big named' opponent then they will make vastly more money from their future fights.
Brook to win this match up if they do get it on in my opinion.