[BostonCard]

There is a correlation, but as you point out, it is not rock solid (note the data only runs to 2015, and thus excludes teams like Tampa Bay and KC a couple years ago).  Still it is worth pointing out the outliers (teams that vastly outperformed their predicted win percentage based on payroll).  The z-score represents the number of standard deviations above or below the mean payroll for that year (which allows you to compare across years).  The ones that stand out:

1982 Angels: 93 wins and an AL West win, in a year they basically spent 2 standard deviations lower than the mean
1994 Expos: Probably the most amazing job of doing more with less; unfortunately that team's year was cut short by the strike.  They were 74-40 at the time the season was cancelled, which would have projected to a 105-57 season had they played a full year.  And they did it with a payroll about 1.75 standard deviations below the mean.
2001/2002 Oakland A's:  These were the teams that gave Beane his reputation (along with the year before and year after, which all resulteed in playoff berths).  The A's had a payroll that was about 1 standard deviation below the mean for those years, and went 102-60 and 103-59.  You might expect a single season run followed by reversion to the mean, but the A's were able to sustain two 100 win seasons (followed by a 96 win season and proceeded by a 91-win season).
2001 Seattle Mariners; 1995 Cleveland Indians and 1998 NY Yankees: Those three teams had above average payrolls (~0.7 SD for Cleveland and Seattle, and  close to 2 for the Yankees), but these were teams with about a .700 winning percentage. Cleveland went 100-44 in a season whose start was delayed, Seattle was 116-46 and eclipsed the A's that season, and the Yankees were 114-48.

BC

[BostonCard]

There is a correlation, but as you point out, it is not rock solid (note the data only runs to 2015, and thus excludes teams like Tampa Bay and KC a couple years ago).  Still it is worth pointing out the outliers (teams that vastly outperformed their predicted win percentage based on payroll).  The z-score represents the number of standard deviations above or below the mean payroll for that year (which allows you to compare across years).  The ones that stand out:

1982 Angels: 93 wins and an AL West win, in a year they basically spent 2 standard deviations lower than the mean
1994 Expos: Probably the most amazing job of doing more with less; unfortunately that team's year was cut short by the strike.  They were 74-40 at the time the season was cancelled, which would have projected to a 105-57 season had they played a full year.  And they did it with a payroll about 1.75 standard deviations below the mean.
2001/2002 Oakland A's:  These were the teams that gave Beane his reputation (along with the year before and year after, which all resulteed in playoff berths).  The A's had a payroll that was about 1 standard deviation below the mean for those years, and went 102-60 and 103-59.  You might expect a single season run followed by reversion to the mean, but the A's were able to sustain two 100 win seasons (followed by a 96 win season and proceeded by a 91-win season).
2001 Seattle Mariners; 1995 Cleveland Indians and 1998 NY Yankees: Those three teams had above average payrolls (~0.7 SD for Cleveland and Seattle, and  close to 2 for the Yankees), but these were teams with about a .700 winning percentage. Cleveland went 100-44 in a season whose start was delayed, Seattle was 116-46 and eclipsed the A's that season, and the Yankees were 114-48.

BC