You can set up and RTA with a laptop for about $100 and probably have something worthwhile. Plus you can use it as an RTA four tuning your system!I wonder at what price point one would have to go in order to get a reputable SPL/sound mic?
This one from Parts express has got good reviews.
Dayton Audio iMM-6 Calibrated Measurement Microphone for iPhone iPad Tablet and Android
I wish I knew someone who had a good one to compare systems. Aha! I'll use my wife's Samsung and my Daughter I phone to test. Why didn't I think of that?. We have five different phones in this family.
Scroll down on this page a little and they list what you need to get an RTA going...
There is also a free program you can use with that equipment..
Room EQ Wizard
Basically what you need is...
$00 Laptop (Assuming you already have one)
$50 Measurement mic
$07 Mic cable
$43 USB mic interface