i have to half way agree with everybody on this one. you can mix your a** off on a $1000 pair of studio monitors and have a mix that sounds like it was eq'd by the man upstairs...on those monitors. then you go and play it on your mom's sony in the kitchen and it sounds like somebody crapped on it. my point is monitors will ABSOLUTELY help your mixes turn out good with less trial and error. BUT, you can also use what you have and test out your mixes on every source you can get your hands - moms radio, your car stereo (the one that bangs), your boys car stereo (the one that only has one speaker and it only works when you hold the electrical tape tight) the wack a** computer speakers at work, everything. once you do that repeatedly your ears will learn what your "monitors"(headphones, whatever) make mixes sound like. then you'll be putting out mixes that can sound good no matter who's playing it.
Before everybody starts jumping on me, monitors are great. i'm just saying, you gotta work with what you got sometimes. let your lack of funds at the time make you better.