I recommend splitting your budget to buy an additional pair of headphones and spend the rest on the best 8 inch driver monitors you can afford (used Mackie HR824s for instance).
In the end you need your work to translate as good as possible to the other studios. This means you need to hear full bandwidth (including sub) and be very sure your listening environment acoustics arn't mucking up your perception of what you're doing.
You don't have the budget to make a solid listening environment or buy a mains and sub system. But you CAN make sure your premixing translates by cycling between some speakers, the Beyers and say some Audio Technica ATH-50s.
You'll be absolutely amazed at how different your stuff sounds between these three setups...and if your stuff sounds reasonably good on all three....chances are pretty good you won't be overly surprised at the mix.
Also keep this speaker axiom in mind - What your speakers are good at is probably what your mixes with lack. In other words if you speakers have better bass than normal speakers...then guess what your mix will lack when heard by the general public.
Here's what we have at Wabi Sabi Sound:
Speaker - General Thoughts
JBL LSR 4326P 5.1 - Forward Mids, can be a bit harsh, nice features
Dynaudio BM15a 5.1 - Silky, Lots of Dynamic range, a little hollow in the mids
Blue Sky Pro Desk 5.1 - Well balanced in a small room, nice features
Blue Sky Media Desk 5.1 - also well balanced, but less detailed
Neumann KH 120s 2.1 - Tight, detailed really need a sub for post
Beyerdynamic DT 770 Pro - smily faced, lots of detail, lots of high end
Audio Technica ATH-50 - Balanced not hyped...break up somewhat easily
Senneheiser HD 380 Pro - Detailed in the mids, not much low end extension or high extension
Grado SR80 - Natrual and Neutral...kinda like listening to speakers, open lacks deep bass