The Williams "FireSights" are NOT illuminated.  They simply incorporate a piece of fiber optic rod, which gathers ambient light and directs it out the front and back of the rod, forming a "lit" circle or bead from the perspective of the shooter.  However, this only works when there is plenty of ambient light to collect.  At night, it is just as dark as a standard sight.

The Trijicon sights use a vial of tritium gas, a radioactive gas that emits light, and has a 12.5 year half-life.  These sights glow actively, and require no external light sources of any kind.  They are expensive because tritium is the most expensive substance on the commercial market.  They're also worth it.

-Troy