Acoustic measurements require the use of a suitable microphone, but not necessarily an expensive one.
In general we suggest that you use a microphone that is specified for test and/or measurement use. This often means a small-diaphragm condenser (or electret) microphone, and you can obtain these microphones for as little as $70.
Note that while there are some recording microphones that may appear to have suitably flat response curves for measurement, that only tells part of the story. Measurement microphones often have much lower sensitivity (allowing for higher inputs exceeding 100dBSPL) and lower self-noise characteristics.
Measurement microphones are also omnidirectional and hence are better suited to capturing reflections at all angles equally well.
Most measurement microphone manufacturers supply calibration data that FuzzMeasure can import to apply corrections to your frequency response graphs automatically, accounting for any deviations from a perfectly flat response.