From http://stackoverflow.com/questions/2714639/weighted-average-with-linq

linq-weightAverage.cs

public static double WeightedAverage<T>(this IEnumerable<T> records, Func<T, double> value, Func<T, double> weight)
    {
        double weightedValueSum = records.Sum(x => value(x) * weight(x));
        double weightSum = records.Sum(x => weight(x));

        if (weightSum != 0)
            return weightedValueSum / weightSum;
        else
            throw new DivideByZeroException("Your message here");
    }

Single pass implementation ->

public static double WeightedMean<TSource>(this IEnumerable<TSource> list,
    Func<TSource, double> valueSelector, Func<TSource, double> weightSelector) =>
    list.Aggregate(
        new { Sum = 0.0, SumWeights = 0.0 },
        (accumulator, l) =>
        {
            double sum = valueSelector(l) * weightSelector(l);
            double weight = weightSelector(l);

            return new { Sum = sum + accumulator.Sum, SumWeights = weight + accumulator.SumWeights };
        },
        accumulator => (accumulator.SumWeights != 0) ? accumulator.Sum / accumulator.SumWeights : 0
    );