I believe this would be a reasonable basis for the crown value/weight:

Crown weight 3 lb [url=http://www.d20pfsrd.com/magic-items/wondrous-items/wondrous-items/c-d/crown-of-swords](using the same weight of the crown of swords) [/url]

Edit: [url=http://www.d20pfsrd.com/magic-items/wondrous-items/wondrous-items/c-d/crown-of-blasting](Crown of blasting has 1 lb so there is a leeway with the weight) [/url]

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If crown made of silver:

market value of silver = 1 lb - 5 gp

Calculation:

silver 5gp(pound) X 25%(average purity)= 1 gp 25sp (ore) ->

(smelting) commercial price goes from 1/2 to 3/4 so +- 60 sp to 90 sp

X 3 (weight) = 1 gp 80 sp to 2 gp 70 sp [for easier numbers 2gp to 3 gp]

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If crown made of gold:

market value gold = 1 lb - 50gp

calculation:

gold 50gp(pound) X 25%(average purity)= 12 gp 5 sp(ore) ->

(smelting) commercial price goes from 1/2 to 3/4 so +- 6 gp to 9 gp

X 3(weight) = 18 gp to 27 gp 
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of course being a crown it could be more valuable the these numbers, even more if you change the metal purity value.

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