expenses.cs

    public class Travel
    {
        public IList<Expense> Expenses { get; set; } = new List<Expense>();
    }


    public class Cost
    {

        public decimal Amount { get; set; }
        public string Currency { get; set; }
        public PaymentType Payment { get; set; } = PaymentType.Unknown;
    }

    public class Expense
    {
        public int TravelId { get; set; }
        public DateTime Date { get; set; }
        public Cost Cost { get; set; }
        public Tip Tip { get; set; } = new Tip();
        public ExpenseType Type { get; set; } = ExpenseType.Unknown;
        public bool IsTaxFree { get; set; } = false;
        public decimal TotalCost => Cost.Amount *  + Tip.Cost.Amount;

    }

    public class Tip
    {
        public Cost Cost { get; set; } = new Cost();
    }

    public enum ExpenseType
    {
        Unknown,
        Food,
        Souvenirs,
        Museum
        //...
    }
    public enum PaymentType
    {
        Unknown,
        Visa, 
        Cash,
        Mastercard
        //...
    }