It was widely reported that Chelsea rejected 55 million+ offer for him from Barcelona so I doubt we'd get him for anything lower than at the very least 60 million+. If they wanted to sell him surely they would have sold him then, especially with us lurking around.
But that might also be the whole point though, maybe Chelsea are waiting for us knowing that we'll , with the clauses we have in place, will be able to fund their deal for Zaha in order to get Willian as you say. They get Zaha without spending a penny net and we get Willian at a (perhaps phony?) discount. Whereas if they'd taken Barcas offer they'd have lost around 20 million net whilst replacing him.
Whilst you could consider it a win-win situation I'd still feel like that's poor business from us though. Chelsea blew the money we paid for Matic on Bakayoko, this time I feel like we'll be on the short end of the stick.