If your mirror is a simple rectangle without beveling or fancy edge work it is cheaper to buy a new mirror. One might want to resilver a simple rectangle if it is antique glass and the desire is to retain the look of old glass. Antique glass is imperfect. It is wavy, may have small air bubbles and beveling is uneven. Patterns other than rectangles are usually expensive and it is much cheaper to resilver.
Some mirrors display fancy etching or engraving.
These would also be cheaper to resilver. Usually the value is not destroyed by resilvering. Remember, you are still using the original glass. Actually,the resale value is usually increased. It needs to be an important piece for there to be a loss in value.