If I understood your question correctly,
decimal total_prize = 0;
foreach (DataRow row in dt.Rows)
{
PurchaseAccountBLL a = new PurchaseAccountBLL();
a.Itemname = row["item_name"].ToString();
a.Id = Convert.ToInt32(row["purchase_id"]);
a.Purchase_Unit_Prize = Convert.ToDecimal(row["unit_prize"]);
a.Purchase_Item_Quantity1 = Convert.ToDecimal(row["quantity"]);
a.Purchase_Item_Discount1 = Convert.ToDecimal(row["discount"]);
a.Purchase_Item_Total1 = Convert.ToDecimal(row["item_total"]);
total_prize += Convert.ToDecimal(row["unit_prize"]);
purchase.Add(a);
}
decimal avg = total_prize/dt.Rows.Count;
-KR