I spent several years mixing in an untreated room (with Yamaha HS50M monitors, which are a good budget option). I've have a properly treated studio room (with good hardwood flooring) for about a year. During all of this time, I regularly switch between monitors and headphones to get a second perspective.
Given that experience, my advice would be this: If you can't afford to treat a room, pick up a quality set of studio headphones and spend the majority of your time mixing on those. I've really come around on the whole headphone vs monitor debate. Monitors are most definitely preferable IF you have the right environment to use them. Otherwise, you're introducing less variables (and have to do less compensating) mixing with a good set of cans. Regardless of your setup, ALWAYS reference, and check your mix on multiple systems.
I'm guessing some people will disagree with me, and that's fine. The thing that ISN'T debatable is the effect of proper acoustic treatment -- it makes a gigantic difference. If you're dead-set on getting a set of monitors, I'd suggest taking some of the budget to invest in treatment. If you are willing to DIY (properly), you can get pretty far with a few hundred bucks.