Three things to consider here.
First, although rumours are saying that MAtS will be our #1 GK next season I'm still not convinced if Lucho and Unzue will really make that decision. MAtS willl probably also miss first week or two of preseason because he was with Germany. Yes, he didn't play in Euro but he has right to rest for 4 weeks like all other players during the summer but maybe he'll shorten his vacations to come back earlier (I actually expect him to do that but you never know). Anyway, Bravo starting preseason later due to Copa America didn't influenced Lucho's decision last season but we'll see what will happen this year.
Second, if Lucho decides to make MAts our #1GK for La Liga matches, will he completely reverse goalkeeping roles so that Bravo will play in CL? I think he'd probably accept that because he'd have a chance to win CL (on the pitch, not just watching from the bench). But if MAts will play in La Liga and CL and Bravo only in CDR, then I could see him having problems to accept that and looking for transfer to another club.
Third, in case Bravo decides to leave we should look at net profit of his sale. Those rumoured prices of around 18-20M seems fair to me because of his age and this is the last chance for us to sell him for that high price. Next summer he'll be a year older with a year less on his contract and most likely relegated to #2 GK role which will all influence his price. But again, those are just rumours. We don't know what offers (if any) did the club and Bravo really get.
So if we sell him (on his wish, as I don't expect we'll sell him if Bravo accepts his new role) we should look at what his possible replacement will cost. If we can sell him for a net profit of around 10-15M (what we'll get for Bravo minus what we'll have to pay for his replacement) I think we should do that. If it's for anything less I think it's more worth for us to keep Bravo for another season or two as a great backup in case anything happens to MAtS. That's unless the club thinks Masip can be trusted as our second GK. I don't and most people here will probably agree with that. Maybe we are unfair to him and he'd be great if given a chance but I'd rather not risk to see that. And based on rumours Lucho who has seen him on training every day for the past two seasons thinks the same.
I don't think Napoli will sell Reina cheaply as he's still their #1 GK, so Diego Alves could probably be the best available option as Valencia is apparently ready to sell him. If we can get him for around 5M and sell Bravo for 20M I'd do it. But it's unlikely.