"Some courts have accepted evidence of the disparity between a fund's costs and returns as providing sufficient evidence of the plausibility that a plan sponsors failed to properly perform their fiduciary duties. [In one] such case ... Judge Sidney Stein denied Citigroup's motion to dismiss ... citing the fact that the plan participants had shown that in some cases the expense ratios of the funds in question were 200 percent of more higher than the expense ratios of comparable Vanguard funds. Judge Stein's recognition of both the impact of a disparity in expense ratios and the legitimacy of Vanguard funds as comparators in 401(k) and 403(b) cases should not go unnoticed."