Concentrate on calibrating your room and you ears, not your monitors.
Unless you have really horsecrap speakers, they should be fine for making a decent mix as long as your room isn't screwing them up and as long as your ears are listening properly.
As far as the room goes, check out Ethan's tips in the studio forum for how to best get rid of bass modes and properly control high frequency reflections for cheap.
As far as your ears, which are far more important than anything else, learn to pay attention to the "translation"; i.e. if my mix sounds like X on my monitors, it will sound like Y on my stereo. Mix fof the translation.
There are already two or more active threads in these forums explaining why "calibrating monitors" is not the recommended strategy.
G.