Yep, I can't think of any place where you use blueprints, with the option of spending diamonds if you don't have any/enough, where substitute cost is anything other than 300 diamonds.
That being said, if you are not in a fellowship that earns a blueprint every week, upgrading Magic Residences or Magic Workshops that are lower than about chapter 8 or 9 is a waste of diamonds and/or blueprints, because of the cost to buy one of those buildings new. So if you have some of them, and they are several chapters behind where you are, it may be more cost efficient to buy new ones, if you have diamonds you don't plan on using elsewhere.