At the moment, i think measurement mic and inexpensive is almost an oxymoron.
I ended up purchasing a kit (url in first post) and building one after finding a company in Australia which sells electrets with a good frequency response. The circuit actually puts out a signal with enough voltage to be recorded using a line level input, that should help to minimise the amount of noise that would otherwise be picked up on the microphone jack on a PC..
The other thing about getting a real measurement mic is that you'll probably have to buy or build all the balanced line receiver equipment: phantom power supply and a balanced line to line level converter.. That would probably either be quite expensive or quite time consuming!
g'luck with your project though, i'll post if i make any progress on mine..