using System;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var obj = new SomethingDto { value = 10M };

        string json = JsonConvert.SerializeObject(obj, new DecimalConverter());

        Console.WriteLine(json);

    }

}

​

public class DecimalConverter : JsonConverter

{

    public override bool CanConvert(Type objectType)

    {

        return (objectType == typeof(decimal));

    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)

    {

        writer.WriteRawValue(((decimal)value).ToString("0.00"));

    }

    public override bool CanRead

    {

        get { return false; }

    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)

    {

        throw new NotImplementedException();

    }

}

​

public class SomethingDto

{

    public decimal value { get; set; }

}

​