Greetings HK.
If I am correct, I believe all Officer's badges were private purchase, bought either by the officer himself or perhaps a gift from a proud relative.
Some were hallmarked, others impressed with the word Sterling and others with nothing to support the fact they were silver.
Then there are those that are silver plated and in some instances these are marked on the rear with a letter P.
If you look at my Essex revamped Album
here you will see the various styles of Officers' cap badges in silver, silver plate, white metal and OSD.
In answer to the question, are they common; they are not scarce bearing in mind the number of years The Essex Regiment has been in existence and the number of Officers that must have passed through over the years.
However, those to The Territorial Battalions will be scarcer, due to the fact of their shorter time span and less number of Officers.
Price wise, they can be anything from £80 or more upwards to an early pattern with hallmark which I have seen with an asking price of £175.
Hope this helps, but if I can assist you further, please ask.
Regards
Brian