your link showed their value approx £650
That is a retail price from a major paperweight dealer. It might be a reasonable amount, but if the item has been in the dealer's stock for some time, it could indicate that the price is too high for the current market. However, in the Charlton Standard Catalogue of Caithness Paperweights (2nd edition, 2004) the set was given an approximate value of £1,200. So, if a major paperweight dealer five years later is only asking about half that, it shows that the "value" is extremely uncertain, or that the "Charlton" guide value was very optimistic.
On the other hand, a good single Caithness weight made by Willie Manson and signed with a WM cane, should have a market value of over £100. So four similar weights ought to be at least
£400 - 500. And a boxed set of four, with certificate, should add a bit more to the market value.
So the Sweetbriar market price is probably about right for current times and could be used as a reasonable idea for an insurance valuation. But if the set was put into auction, even through a major auction house, it might struggle to make £300-400 hammer price, with deductions for commssion and tax making it seem "very cheap"!