The size of the fish is listed as 370mm SL in the Cat-eLog. Fishbase lists 49cm TL. So around 45-50cm seems like a reasonable max size.
As to what price they fetch, that would depend a lot on where you are, so if you fill in your location, that would at least be a help on suggesting something.
They are not very common, that's for sure. But that in itself isn't going to mean that they are "valuable" - they may just not be very commonly on the market because there's little demand for them... I couldn't find one for sale on any of my usual search-places, nor by Googling. Apparently they are common in the native environment (according to Jools in this
article)
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Mats