Simplifying calculation with min value in C#
Well, the most simple way would be to use Math.Max() like so  

result.Calculation_VK = Math.Max(input.km_past_year * 0.040m, 300) * Constant.MWST  


which could be as well in a method like  

public static decimal CalculateVK(decimal pastYear, decimal factor, decimal minValue)
{
    return Math.Max(pastYear * factor, minValue) * Constant.MWST;  
}  


and be called like  

result.Calculation_VK = CalculateVK(input.km_past_year, 0.040m, 300);