How long do the electrodes last and what would it cost to replace them if I had to after the warranty had expired?

Tyent offers a 20 year warranty on their electrodes and is the only company that does so.  They did not offer information on the cost of replacing the electrodes as they guaranty them for 20 years.

KYK guarantees the electrodes in their units for the life of the machine.  When the KYK units are released, they are tested on a 24/7 continual flow basis and run for well over a year of use (without powering them down or stopping the water) to allow for accurate testing of the durability of the plates and components. 

The Genesis is proven to maintain a 10+pH and -250 or better ORP for over 30 years of use, based on an 8 gallon per day usage (which is quite heavy usage compared to what the typical household would normally use).

As for Jupiter products, the most someone has been charged to completely rebuild a machine out of warranty is $150.  It may go up to $200 or perhaps $300 at some point due to fluctuations in the dollar and the cost of spare parts, but right now, if someone needed a new cell in an old machine that was not under warranty, it would cost $150, including the cost of shipping it back to them.  This includes replacing everything, the circuit boards, water cells, whatever there is that needs to be replaced.  The water cells cannot be repaired, so they are just tossed out and replaced.

As for Life products, unfortunately we do not have this information available at this time.