A calibrated EMM-6 from cross spectrum labs is $75 and an ART Dual Pre (no personal experience but I've read that others have used them with success) is $79, total $154 plus postage.
http://www.cross-spectrum.com/measurement/calibrated_dayton.html http://www.amazon.com/Art-USBDualPrePS-ART-USB-Dual/dp/B002KEAT78Personally I would spend a little more on the USB interface and get something like a Focusrite 2i2 which is $119:
http://www.amazon.com/Focusrite-2i2-USB-Recording-Interface/dp/B005OZE9SAFor even lower total price, there are the USB mics from PE (UMM-6) and miniDSP (UMIK-1). I believe Herb from CSL offers (or is about to offer) calibrated versions of both, apparently around $90. The USB mics are less flexible than having a separate mic and interface, drawbacks being:
* Their sensitivity is low, so you need to have pretty good acoustic levels and even then the measurement programs may complain that the level is too low
* The electronics tend to be noisy so again you need to keep measurement levels up
* They only run at one sample rate, so you may in some odd cases run into measurement program in/compatibility, and you'll never be able to run them at higher rates
* You can't do timing measurements which require a separate loopback connection
* You can't calibrate the frequency response of the electronics
These are mostly minor issues. But added up, they are significant (I think - YMMV). It's a bit like buying power tools (I use this analogy since I'm currently going through this very exercise for the purpose of house repairs...). You could buy a couple of cheap cordless tools for a job, use them, and in the process figure out what they don't do well and then finally realize that to do the job right you are going to have to buy the good tools! Would have been cheaper to get the good ones in the first place....
http://www.cross-spectrum.com/measurement/calibrated_dayton.html http://www.amazon.com/Focusrite-2i2-USB-Recording-Interface/dp/B005OZE9SA