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public class InvalidType
{
}

private Type GetPropertyType(string propertyName)
{
    Type type = typeof(InvalidType);
    try
    {
        type = typeof(MyItemClass).GetProperty(propertyName).PropertyType;
    }
    catch (Exception)
    {
        // eat the exception
    }
    return type;
}

public int GetMonthValues(string column, int year, int month, out int total)
{
    double average;
    double dblTotal;
    int count = 0;
    total = 0;
    count = GetMonthValues(column, year, month, out total, out dblTotal, out average);
    return count;
}

public int GetMonthValues(string column, int year, int month, out double total)
public int GetMonthValues(string column, int year, int month, out int total, out double average)
public int GetMonthValues(string column, int year, int month, out double total, out double average)

private int GetMonthValues(string column, int year, int month, out int intTotal, out double dblTotal, out double average)
{
    int count  = 0;
    intTotal   = 0;
    dblTotal   = 0d;
    average    = 0d;
    // the GetItems method is just a linq query that retrieves 
    // all of the items with the specified month/year. It certainly 
    // doesn't need to be a separate method, but that's where my 
    // laziness kicks in, and I just left the merthod as is. :)
    List<MyItemClass> itemList = GetItems(month, year);
    count = itemList.Count;
    Type propertyType = GetPropertyType(column);
    if (propertyType == typeof(int))
    {
        intTotal = itemList.Sum(x => (int)(x.GetType().GetProperty(column).GetValue(x, null)));
        average = ((double)intTotal / count);
    }
    else if (propertyType == typeof(double))
    {
        dblTotal = itemList.Sum(x => (double)(x.GetType().GetProperty(column).GetValue(x, null)));
        average = (dblTotal / count);
    }
    else
    {
        throw new Exception("Specified property name either does not exist or of an unsupported type.");
    }
    return count;
}