I agree, that discussion will come soon if indeed Spence happens to be the winner in this fight. For now, let's talk about Ugas and Spence as this is also an anticipated and interesting fight to see.
There's no other way. Crawford just needs to accept that 40%. He is the one hungry to fight Spence and the latter didn't even bother if the fight against him will happen or not. Spence will just go on his plan of moving up if Crawford won't agree with the purse split.
Spence is the big reason why the fight will generate more money and not Crawford. After he finished Spence, then demand more on his future fights, easy as that. For now, to make that fight happen, Crawford should just accept that Spence is really the King of Entertainment compared to him.
But this discussion can only happen if Spence will pass Ugas. What if Ugas won? It will now be Ugas vs Crawford and the purse split is 100% 50/50.