Expensive landing light bulbs from the FBO are expensive because he
has to pay way more for that bulb. In Canada, and I imagine it's the
same in the U.S., all aircraft parts have to be traceable all the way
back to the manufacturer, and the standards the manufacturer has to
meet are a bit tougher than the auto manufacturer's standards. So while
it's the same bulb, the manufacturer has to test one in ten instead of
one in a hundred (or some similar difference), and has to provide
certification paperwork for every bulb destined for aviation use. All
that extra time adds up. Add to that the liability factor: a dead
landing light on a 4X4 isn't quite the inconvenience or safety risk
that it is on a Navajo.
The Canadian aviation regulators have been considering "Commercial
Equivalent" legislation for some time now. They recognize that the
prices we pay are exorbitant, for little good reason, and are trying to
see if safety could be maintained while letting us use bulbs or
alternator belts or wheel bearings from NAPA or whoever. Their biggest
concern is the cheap knock-off from China that sometimes finds its way
into packaging that looks just like GE's or Timken's.

Dan