Jaroslav Halak just signed a 4-year, $15M contract that sells his three remaining RFA years along with his first UFA year. Halak's established talent level is a 925 even-strength save percentage, compared to a 909 replacement level, which, if he plays as much as Evgeni Nabokov or Marty Turco did over the last five years, will be worth approximately 80 goals above replacement, or 13.4 wins. Thus St. Louis is paying at least $1.1M per win - more if Halak gets seriously injured or becomes ineffective.
From the first day of free agency, it appears that the current price for UFA wins is $780k. So Halak is being overpaid, but not by all that much. But remember that he's an RFA, and RFAs typically leave 40% on the table, so his contract is equivalent to St. Louis giving a UFA $5M-$5.5M per season. His performance isn't significantly better than Evgeni Nabokov (925 for Halak vs 922 for Nabokov) and it seems like Nabokov might have trouble getting much more than $3M on the UFA market.
Overall, St. Louis limited their downside: they didn't significantly overpay for Halak, either in years or salary, and all they gave up was Lars Eller, who is even-odds to produce zero for Montreal over the course of his career. If we go through the median numbers:
Halak contract: $15M
Halak's current market value over four seasons:$10.45M
Median value of Lars Eller: 0 (average value is $2M-$3M)
At present, St. Louis is costing themselves about $4.5M with this deal - but Halak's contract could become a better value as the goaltender market recovers. There were some very bad possible outcomes here and the Blues largely avoided them.