You can´t really get flat response speakers that way. I´ll rather you buy real Nearfield Monitors. But one way to cut real cost is too use normal hifi speakers. Hifi speakers does not have a flat response and tend to boost the bass. The high´s would also be messed up, but that is depended on your speakers. Alongside, get some small computer speakers, these really boost up the highs and lags in bass. Monitor these two alongside and also get a headphone. Using headphones, the highs and the lows get boosted as well. With the three alongside as comparison, you would be able to understand which to boost and which to cut. After much trial-n-error, sooner or later you would be able to capture the right mix of frequencies, which is the point of the whole story. A lot of monitoring is about the ears and understanding your equipment.