To provide some examples of how this works, lets talk about this couch.
As I have said before (quoting a friend, Sarah Miecielica) "theatre is art, on a schedule and with a budget." We would all like to have more money, more time, and more resources, but that is not the business that we are in. It is immensely helpful for the entire production team to come in with realistic expectations and, together, to develop a sense of which design elements are priorities and which are less important. That is not to say that miracles can't happen. I have experienced more than my share of theatre magic. Sometimes things that you need show up in alleys, sometimes a kind store owner or stranger lets you rent or borrow or buy something for far less than its actual value, sometimes a random friend comes out of the wood work to reveal that "oh, my dad has one of those in his garage he's being trying to get rid of," but those magical moments should be appreciated as the lucky gifts that they are. Plan-A should never be luck. It is always best to know, as early as possible in the process, what we will do if nothing lucky comes our way, and if we end up having to pay current market price for everything we need.