There is a simple solution: Room EQ Wizard (http://www.hometheatershack.com/roomeq/) and a RS sound level meter. The software is free and the RS meter is cheap. It will have some FR issues at the low end but is adequate.
The next steps up are to:
1. Get a better microphone for REW. See MiniDSP.
2. Get an alternative mic/software package. See XTZ and Omnimic.
The next steps up are to:
1. Get a better microphone for REW. See MiniDSP.
2. Get an alternative mic/software package. See XTZ and Omnimic.