short answer: the fair price of a company (assuming that you keep running the company) is its stand alone value (profits) PLUS synergies that can be expected over a certain investment horizon. Discounting is almost always disregarded in case interviews.
The interesting thing is that this principle thinking frame is not only true for M&A cases, but for 90% of all cases that you will ever encounter (market entry, new product, capacity expansion, licence purchase, etc. etc.). It is always about value creation! If you learn to rigorously think according to the principles of value creation, the typical case frameworks from Case in Point etc. become practically obsolete.