The MDR-7506 isn't especially accurate, but it is a very practical tool for use on location. For mixing, you'd be far better off with a pair of open-back headphones like the AKG K701 or the Sennheiser HD 650.

Speakers are inherently more expensive to manufacture, so they lag well behind headphones in terms of price-to-performance. The other crucial factor is the acoustic quality of your listening room, which in most cases is a far more significant factor than the quality of your monitors. Rather than buying expensive monitors, you're usually better off buying inexpensive nearfield monitors like the Presonus Eris and spending the rest of your budget on bass traps and diffusers.