The price of Silver fluctuates every so often and it has pretty much doubled over the past 10 years or so. Typical prices (as of 2009) are about $21 - $25 for a set of 5" - 6" length 14 gauge wires.
The silver should always be a MINIMUM of .999 (99.9%) pure.
The use of .9999 (99.99%) pure silver is not necessary but it will give a slightly purer finished product. It will cost more, however.
There are a large number of companies making colloidal silver generators and they use a wide variety of sizes and shapes, including wires, rods, and strips. Always check to make sure you are purchasing the correct size, type and length to fit your particular silver generator.