At the same age, Duncan Keith returned Caleb Jones and a 3rd round pick. He had1.5 less years on his deal (2 years vs. 3.5 years) and ~600k less AAV (5.5M vs 6.1M). That would be the ceiling. Lower-lineup RFA-aged player + 3rd.
Montreal is probably the favorite landing spot if it were to happen, so the equivalent I guess would be Struble + 3rd. Don't expect more if it does go down, which is why I think it won't. Letang is probably more valuable as a tank commander and vet for the eventual youth movement to lean on. Contract isn't a problem for Pittsburgh with what the team will look like over the next few years.