select
 SP.ID,
 round(iif(@export = 0, SP.CENA_FSO, SP.CENA_EXP) / coalesce((select first 1 KV.KURS from KURS_VALUT KV join S_VALUT SV on SV.ID = KV.ID_VALUT where SV.N_VALUT_KR = @valut order by KURS_DATE desc), 1.00), 2) CENA
from S_PROD SP

var export = 0;
var valut = "RUB";         

var list = from sp in sProd
	let maxDate = kursValut.Where(w => w.ID_VALUT.N_VALUT_KR == valut).Max(p => p.KURS_DATE)
	let kurs = (decimal)kursValut.Where(w => w.ID_VALUT.N_VALUT_KR == valut && w.KURS_DATE == maxDate).Max(p => p.KURS)
	select new { 
	   sp.ID,     
	   //CENA = 1.34,
	   //CENA = Math.Round((export == 0 ? sp.CENA_FSO : sp.CENA_EXP) / kurs, 2, MidpointRounding.ToEven),
	   //CENA = ((decimal)(export == 0 ? sp.CENA_FSO : sp.CENA_EXP) / (decimal)d),
	   //CENA = (decimal)(export == 0 ? sp.CENA_FSO : sp.CENA_EXP) / (decimal)d,
	   //CENA = (decimal)0 / (decimal) ku  ,
	   //CENA =  Math.Round((decimal)sp.CENA_FSO / (decimal) ku0, 2),
	   //CENA = sp.CENA_FSO* (decimal)kursParam,
	   CENA =  Math.Round(sp.CENA_FSO,2),     
	};

           var l = list.ToList();
            foreach (var item in l)
                item.CENA = Math.Round(item.CENA, 2);

let kurs = kursValut.Where(w => w.ID_VALUT.N_VALUT_KR == valut).OrderByDescending(o => o.KURS_DATE).Select(p => p.KURS).FirstOrDefault()

   public class ValutSiteItem
    {  
        public string N_VALUT_KR { get; set; }
        public string N_VALUT { get; set; } 
    }
    public class ValutSite 
    {       
       public List<ValutSiteItem> Select()
       {
    ...

           var list = from sv in sValut
                      orderby sv.N_VALUT_KR
                      select new { sv.N_VALUT_KR, sv.N_VALUT};                                   

           return list.ToList(); // тут ошибка преобразования 
       }          
...

  public class ValutSiteItem
    {
        public string N_VALUT_KR { get; set; }
        public string N_VALUT { get; set; }
    }

	List<ValutSiteItem> listValutSite = new List<ValutSiteItem>();    

	string sql = "select N_VALUT_KR, N_VALUT from S_VALUT order by 1";

	string connString = WebConfigurationManager.ConnectionStrings["connect"].ConnectionString;

	using (IDbConnection db = new FbConnection(connString))
	{
		listValutSite = db.Query<ValutSiteItem>(sql).ToList();
	}       

      List<ValutSiteItem> list_ret = new List<ValutSiteItem>();
    
           foreach (var item in list)
           {
               ValutSiteItem vi = new ValutSiteItem();
               vi.N_VALUT_KR = item.N_VALUT_KR;
               vi.N_VALUT = item.N_VALUT;

               list_ret.Add(vi);
           }

           return list_ret;

     var list1 = from sv in sValut
                      orderby sv.N_VALUT_KR
                       select new ValutSiteItem { sv.N_VALUT_KR, sv.N_VALUT };

           IEnumerable<ValutSiteItem> list2 = from sv in sValut
                       orderby sv.N_VALUT_KR
                       select new ValutSiteItem { sv.N_VALUT_KR, sv.N_VALUT };