The league average save percentage the last two years has been .915.
There were 20 goalies in the league this year who had an AAV at or above $4.5 million. Only 12 of those 20 had a save percentage above league average (.916 or higher), and 3 of those guys didn't manage to play in at least 40 games (Price, Mike Smith, and Halak). That's 9 of 20 who managed to play 40+ games at .916 or higher. The list of guys who had .915 or worse included Bobrovsky, Rinne, Rask, Ward, Lehtonen, Varlamov, Howard, and Niemi.
The next 20 goalies in decreasing order on the AAV list takes us from $4.33 million down to a cap hit of $1.45 million. Among them, 12 had a save percentage above league average, with all but 3 of them (Lehner, Neuvirth, and Berra) playing in 40+ games. Again, that's 9 of 20 who managed to play 40+ games at .916 or higher.
There are 5 goalies from the 1st group of 20 still playing now (Rinne, Holtby, Bishop, Lehtonen, and Niemi). There are 3 goalies from the 2nd group of 20 still playing now (Jones, Greiss, and Elliott)...4 if you include Allen, who hasn't actually played. The last goalie still playing now is Murray ($628k AAV) for PIT, filling in for MAF (a 1st group guy who has been injured all playoffs).
Coincidentally, a high-priced goalie is going against a low-priced goalie in each series (Holtby vs Murray, Bishop vs Greiss, Rinne vs Jones, and Lehtonen/Niemi vs Elliott/Allen). The only high-priced goalie currently leading their series is Bishop.
This isn't exactly a rigorous scientific study, but what I'm hoping to suggest is twofold: 1) You don't need an "elite" high-priced goalie to be successful in the playoffs. 2) It isn't a particularly efficient use of resources to sink a lot of money into the goaltending position if you have reasonably good lower-cost alternatives available to you. You tend to get better bang for your buck investing that money elsewhere on your roster.
Since the Blues do have a reasonable lower-cost alternative available to them in Allen, I don't think they should pursue Bishop at all. I don't even think they should re-sign Elliott if he's going to command $5+ million over significant term. I'd just roll with Allen and build the best possible team around him that can be built with the savings.