public class SelectedVariation
{
    public int VariationID { get; set; }
    public int VariationValueID { get; set; }
    public decimal VariationCoefficient
    {
        get
        {                    
            return VariationValueID;
        }
    }
}

public ActionResult GetVariationCoefficient(int variationId = 0, int variationValueId = 0)
{
    List<SelectedVariation> selectedVariations;

    decimal Price = 1;

    if(Session["SelectedVariationsList"] == null)
    {
        selectedVariations = new List<SelectedVariation>();
        Session["SelectedVariationsList"] = selectedVariations;
    }
    else
    {
        selectedVariations = (List<SelectedVariation>) Session["SelectedVariationsList"];
    }
            
            
    if (selectedVariations.Exists(x => x.VariationID == variationId))
    {
        selectedVariations.FindAll(x => x.VariationID == variationId).ForEach(
            x => x.VariationValueID = variationValueId);
    }
    else
    {
        var v = new SelectedVariation();
        v.VariationID = variationId;
        v.VariationValueID = variationValueId;
        selectedVariations.Add(v);
    }

    foreach (var v in selectedVariations)
    {
        Price = Price * v.VariationCoefficient;
    }

    return Json(new { coefficient = Price }, JsonRequestBehavior.AllowGet);
}