You may already have the information, but your Highland Light Infantry plaid brooch can be dated based upon the configuration of the crown at the top of the design. The crown is known as a "King's Crown", signifying King Edward, who succeeded Queen Victoria January 22, 1901, and for purposes of certain Regimental insignia, the "King's Crown" continued in wear through the reigns of Kings George V and VI, until the accession of Queen Elizabeth II in 1952.

Accordingly, your HLI plaid brooch is of the design in wear from 1902 to 1952, through both World Wars. Further dating could be accomplished by reference to the battle honours set forth in the scrolls around the inner ring.

See Badges of the Highland and Lowland Regiments (Including Volunteer and Territorial Battalions), Bloomer, East Dulwich, 1982, pp. 51-63.

The approximate value you state is fairly accurate, but it could fetch higher at auction depending on current demand.

As to the question of wearing it, you have a sampling of opinion here already.

As a piper, and reenactor of Scottish regiments, I wear Regimental insignia while portraying a particular regiment.

I take meticulous care that all elements of material culture are correctly represented, that things from other regiments are not mixed in, and that the articles included all are appropriate to the same time period--in other words, no Edwardian, or King's Crown belt locket set when portraying a Victorian soldier, whose locket set would have a "Queens' Crown", no buttons from the French and Indian era on kit worn representing the American Revolutionary War time period, and so on for EVERY bit of kit. In a reenacting context, wearing your brooch in a pattern that pertained from 1902 to 1952 with an 18th century style Great Kilt would cause the TARDIS to spew...

That being said, there are a number of vendors that sell wonderful, and not very expensive, plaid brooches that would be wonderful with a Great Kilt. And generally a lot less expensive than an original Regimental pattern....

Whatever you decide to do with your HLI brooch, start a collection of HLI brooches and badges, heave it in a drawer...enjoy! Be well!