So... your basis for comparison is flawed from the beginning, as you're forced to acknowledge that Eller wasn't the best centre on his team, either, and I challenge you to devise a linear scale that relates "replacement value" among goalies to that among depth centres league-wide.
Most teams use 4 centers per game. Most teams also use only 1 goalie per game.
Was Halak an upper-quartile goalie for the last 3 seasons as a whole? (roughly equals to a no. 1 center) No
He probably even not an average goalie.
Was Eller an average NHL-player as a whole for the last 3 seasons? If he wasn't, he was darn close to it.
Did Eller had ANY upside when we got him? Sure
Is this exactly why we got him? Sure
Did Eller commended a 4M salary per year when we got him? No
Does he even make half of that at the moment? No
Was Eller worth more than his salary last season? Sure
Will he be this year? Hopefully, or else we'll suck.
Did any goalie signed in the bargain-bin would have made better than Halak for the last three seasons? Not every of them
Is Lars Eller a better player than, say, Alex Steen at the moment? Yes.
Would the Blues be a better team with Eller instead of Steen, considering Halak is arguably their 3rd stringer right now? Yes
Did Halak was better than Price for the three seasons? No, he wasn't.
Are we better off with Lars Eller as a 2nd center than with David Desharnais? Probably
In case the answer to the above-quesiton is no, are we better off with Lars Eller as 3rd Center compared to ANY of our other players, EXCLUDING Galchenyuk? ... Certainly.