I use an Audiomatica CLIO Pocket system that comes with a calibrated Omni mic. Parts Express sells them for $525.00. I use it for a lot more than just speaker measurements.
If you want to get started, I would recommend reading about gated time speaker measurements, download Room EQ Wizard and get a Dayton Audio UMM-6 calibrated mic from Parts Express. If the sound card or DAC on your PC is a good one it will work for in home frequency response measurements.