Alexis was the Italian player of the year, at Barca he's played well(3rd highest assists in the squad). Clubs can take account that Barca isn't build around him. ~25M is not ambitious, there'd be a few clubs that would be willing to buy him.
Pedro is on course to break Spains all time annual goalscoring record. The kind of player that can score a crucial goal in a Champions League quarter-final. 20M+ for sure.
Villa is Villa, 10-15M is about right, only problem is his wages maybe, but there'd still be plenty of teams willing to have him. Half the teams in England would be willing to buy him.
Alves is still only 29 and has a high fitness level. If you have a reasonable defensive left-back why not pay 15-25M for the best attacking right back in the world?
Valdes - He's got a good 5 years ahead of him easily, is Spains second choice probably behind Iker, 10-15M if they sell him this summer.
In terms of potential purchases I see the following options:
Neymar 40M
Lewandowski 20-30M(last year of contract and Dortmund would probably prefer to sell to Barca than Bayern so 20M might be possible)
Reus 25M+(Dortmund payed 17M for him)
Aguero ~35M(I don't see Aguero at Barca though tbh)
Suarez 30-40M(the biting incident may have lowered his value)
Piszeck ~15M
Hummels ~30M
Kompany ~30M
I don't think Falcao is possible. I don't see Cavani happening either, too expensive for the risk of another failed transition. I don't see RM players like Higuain and Ramos coming. I don't think PSG would sell Moura or Lavezzi. Reus/Lewandowski are probably the best affordable replacements for Villa, although I expect Reus to stay at his home club. But he does have the perfect profile I think to fit into the current system.