Whether you are a songwriter, producer, or a mastering engineer you need a set of speakers to reproduce what you have created or are creating. Depending on which category you place yourself in, on what level of work you are involved in and what is your budget are the things you need to consider when choosing a new pair of studio monitors. We are going to stick with near fields and put them in three categories based on price.
Budget-friendly, under 500 EUR a pair
These are for songwriters, producers, and mixing is possible with them, it just takes longer and is a bit of a hit or miss situation. In this price range not, all frequencies are reproduced in a balanced way and for critical listening it can be misleading. Songwriters however could get away using even cheaper sets of monitors, because his or her only real goal is to create emotion and one does not need to dig into specific frequencies to make that happen in the writing process. It may vary depending on the style of music though; an EDM track can heavily rely on specific frequencies and sounds to create a certain mood.
Mid-Range, under 1500 EUR a pair
Emotion, as the element that makes or breaks a song, is also the top priority for producers, but here he or she also needs to understand what goes on at different frequencies and even the transient response of the speaker can influence the producer while choosing the correct sounds and instruments for a song. A studio monitor pair in the range of 500 to 1.5k is where the sweet spot is at. With these you can comfortably make a mix happen and these monitors can easily be used in a pro studio as one of the monitor sets. You would still need a different character monitor to complement these and get a different perspective, but these can easily be used as your main workhorse set.
Mastering level
Here starts a different playground and the toys get way more expensive real fast. If you have the funds, then getting a pair of monitors for 5k is by no means an overkill for anyone who is working on audio or likes music in general. These really sound good and yes; you can hear the difference clear as day. The deep, detailed frequency response, the transient response, the stereo field, and so on. Of course, all of this is a necessity when working in mastering where you are the final safety net for a song before it hits the world and you need to get it right, you cannot make compromises and you cannot miss anything.
Conclusion
You cannot take a 200 EUR set of studio monitors and call yourself a mastering engineer, you just would make too many mistakes and would not even know that you have made them, because you would not hear the problems. You could get a mid-level set or even a budget set, do the room correction, and really learn the speakers; how they sound, what are the weak points and compensate, and have multiple listens with different systems before sending out your masters. Higher-quality monitors make post-production so much more enjoyable and easier; you would just hear all the mistakes and fix them accordingly.
Monitors of course are just part of the equation. A decent room with acoustical treatment for the higher frequency issues and some form of room correction before the speakers for the low-end frequencies really help you get the most out of your monitors, whatever the price level.