As large scale power grids add generation resources that are interconnected via DC interfaces and power electronics (including wind turbines) and retire thermal generators, the total amount of inertia in the machines connected to the grid declines, and the grid becomes "weaker". Wind turbines could provide synthetic inertial support, but that comes at some cost of power provision to the system. Using the 16-machine 5-area dynamic equivalent of the New England-New York system, we model the system benefit of synthetic inertia in terms of improved steady-state frequency following the loss of a generator or increase in total load under three wind scenarios, and trace out opportunity costs for a DFIG wind farm providing synthetic inertia. We find that, for a given deviation from MPPT, the wind farm is able to provide a greater level of inertial support to the system, as wind penetration declines. Further, for larger deviations from MPPT we observe decreasing returns in terms of provision of inertial support across all three wind scenarios.