Not expensive. Pretty cheap, really. Two resistors, three caps and a simple EC probe. That's all it takes. The magic happens mostly in software. It does need three pins. Expected price USD 5-10 including probe. Cheap enough I can probably forego the kickstarter route even. Also working on building a version with I2C interface version that includes temperature sensor, so it can be sold calibrated and with temperature correction all ready to go. Ideally also adding a pH sensor to it, but that's for a little later.
You need to get your resistor to be about the same as the resistance your probe sees. You can't calculate this, you have to measure this. Be aware that after 1 ms of applying current in one direction the resistance you measure starts to change, so your measurements are off.
The easiest is to calibrate using an NaCl solution (kitchen salt). A 1000 ppm solution gives an EC of 2 mS/cm. Just see what resistance you get.