Equivalent of C# 'readonly' for an MS SQL column?

Question

Imagine there is a Price column in Products table, and the price may change.
I'm fine with it changing but I want to store the original Price value in another column.

Is there any automatic way MS SQL server may do this?

Can I do this using Default Value field?
Do I have to declare a trigger?

Update

I tried to use Price to simplify the question but it looks like this provoked "use separate table" type of answers.
I'm sorry for the confusion I caused.

In the real world, I need to store a foreign key ID and I'm 100% I only need current and original values.

Update 2

I got a little confused by the different approaches suggested so please let me explain the situation again.

Imaginary Products table has three fields: ID, Price and OriginalPrice.
I want to setOriginalPrice to Price value on any insert.

Sometimes it is a single product that gets created from code. Sometimes there are thousands of products created by a single insert from a stored procedure so I want to handle these properly as well.

Once OriginalPrice has been set, I never intend to update it.

Hope my question is clearer now.
Thanks for your effort.

Final Update

I want to thank everyone, particularly @gbn, for their help.
Although I posted my own answer, it is largely based on @gbn's answer and his further suggestions. His answer is also more complete, therefore I mark it as correct.

Answer 1

After your update, let's assume you have only old and new values.

Let's ignore if the same update happens in quick succession because of a client-code bug and that you aren't interested in history (other answers)

You can use a trigger or a stored procedure.

Personally, I'd use a stored proc to provide a basic bit of control. And then no direct UPDATE permissions are needed, which means you have read only unless via your code.

  CREATE PROC etc
  ...
  UPDATE
    MyTable
  SET
    OldPrice = Price,
    Price = @NewPrice,
    UpdatedBy = (variable or default)
    UpdatedWhen = DEFAULT --you have a DEFAULT right?
  WHERE
    PKCol = @SomeID
    AND --provide some modicum of logic to trap useless updates
    Price <> @NewPrice;

A trigger would be similar but you need to have a JOIN with the INSERTED and DELETED tables What if someone updates OldPrice directly?

  UPDATE
    T
  SET
    OldPrice = D.Price
  FROM
    Mytable T
    JOIN
    INSERTED I ON T.PKCol = I.PKCol
    JOIN
    DELETED D ON T.PKCol = D.PKCol
  WHERE
    T.Price <> I.Price;

Now do you see why you got jumped on...?

After question edit, for INSERT only

  UPDATE
    T
  SET
    OriginalPrice = I.Price
  FROM
    Mytable T
    JOIN
    INSERTED I ON T.PKCol = I.PKCol

But if all INSERTs happen via stored procedure I'd set it there though....