Anyone selling anything other than standardised precious metals or gemstones will be puzzling over this question endlessly.  How do they price items to sell and make some money but not over price something so it never sells?
If you have an ounce of 18kt gold whether it is broken vintage gold jewellery or gold in modern jewellery, you have a measurable amount of a standardised product which you can sell for a the agreed rate for gold on that day.

If however you have a very rare piece of vintage costume jewellery which was designed by Archibald Knox for Liberty as a part of the Arts & Crafts movement then the answer is not so clear.  The materials were standard, in every day use and the value came in the design and wokmanship, rather than the value of the matierials.
The value for a piece of vintage costume jewellery like that comes from its design, rather than its intrinsic value.  Another factor adding value in the example above would be knowing that the item was made by a certain designer.
If you were a collector of vintage jewellery from Liberty and specialised in collecting pieces designed by Knox, or the English Arts and Crafts movement, then the perceived value of that item to you would be vastly higher than for someone who knew nothing about either the make or the designer or the design period.
Someone unaware of the Arts & Crafts Movement seeing the brooch featured in this article would simple see a fairly small hammered copper brooch, with the construction being very obvious and with no gemstones or gold.  How could it be worth anything?
So it seems the answer to the question “How Much Is It Worth?” is really that it is worth as much as someone will pay for it, and no more.
It is simply no good to say well two years ago I paid £700 for this rare Kramer parure and so now today, it must be worth much more.
Having started my website to downsize my collection of vintage costume jewellery, mostly inherited from family, it is extremely difficult to put a price on items.  Some things I know are exceptionally rare  (like the c1800 tortoiseshell brise fan shown above), but other things are more mundane but still collectable.
As a result I first research what others are giving as prices and will peg mine somewhere near.  The rub is that I know for sure that 99% of the vintage
costume jewellery I am selling is unique.  Whilst many may have been produced, only one or two survive.

The long amber bakelite necklace shown above sold recently for under £100  –  the guaranteed Jakob Bengal necklace shown below is still for sale and all offers will be considered at one eighth the price stated on a Jakob Bengel dealers site.

I have everything from very early Bohemian to signed Har and Boucher pieces. Kramer and Weiss are plentiful as are guaranteed Delizza & Elster pieces.
Visit and snag a bargain.  Hopefully we wont be locked in a recession for too long, so grab it while you can.