The problem with drying them using a hair dryer is the warm air could hold quite a lot of water vapour.

When I had this problem, I waited until a day when the relative humidity was low (you need some way of measuring it). Remove the bulb and holder, use the hair dryer on warm, blowing it inside the headlamp. Do this for some time after all the condensation disappears. Turn the hair dryer to cold and continue to blow it in the headlamp until everything is cool. This way you ensure the headlamp is filled with cold, dry air which is capable of holding less vapour than warm air.

This method worked for me for several years, but I notice condensation's starting to return on the inside of the lens on particularly cold and wet days. I presume the air inside the headlamp expands and contracts when the lights are used and the air inside gradually becomes more humid due to the vent in the bulb holder. I guess there has to be a vent somewhere in the headlamp to allow the air to expand and contract as it warms and cools.

Unless of course you do have a hole in headlamp that's allowing water in.

Seems to be standard lamp manufacture now to be clear plastic bonded on, why they are so expensive to replece i don't know, one thing is certain they don't last decades like real traditional headlights with glass lenses.

They are expensive because one can't buy a cheaper one anywhere else !