The short answer is that you have to educate them and if there expectations are much more than what is possible you need to explain the consequences of such scenarios to the producers or directors and then work from there. Generally the director (If they are not also the producer) will be on board with making the film or project the best possible in all areas, so you have to out sell them on the ideas and pitches from other departments essentially.
Time > Money > Quality > Time > Money….. Depending which outcome you want you need the other too. If you want to make a lot of money on a project then you need time and quality to be able to market it. If you want quality you need time and money. If one is missing then you have to compensate more with another.