1. Log into and go to the "my positions" section
2. Click on the blue button "Add to workspace"
3. Click on the "Add/remove columns" button (on the left to Market and looks like 3 horizontal lines)
4. Select the column "Book Costs"
5. You should now see the Book Costs column in your positions, if you like on the value of the stock, you can edit it.
Note: I had problems where I was able to edit the value but it never saved, this is a browser problem. Use a different browser or clear your browser history/cache.
Note: These instructions are accurate at the time of this post. IG may change the user interface in the future which could change the way this is done.