Some ion-selective electrodes can be expensive (starting from $500). Is there any relatively inexpensive do-it-yourself alternatives that would still give very accurate results? How does one make an ion-selective electrode?
Ion selective electrodes are a group of electrodes that are very different from each other.
The common bits are, a selective membrane, a "sensing" electrode and a reference electrode to measure against.
The details of the membrane and sensing electrode depend highly on what selective electrode you are trying to build.
Here is an old patent on a Cl selective electrode for reference: http://www.google.com/patents/US3740326
Regarding getting accurate results with a DIY: this really depends on which electrode you are trying to build. In the Cl example, I don't see any reason why a DIY would perform worse.