Last 'season', meaning last year they were 360 NT$/kg, now the season just started and there is a shortage on mussels from Holland, the 'flesh weight' is still low 15-20% of the mussel weight were it normally should be like 30%. But the guys in Holland are trying to save what they can by importing mussels from Denmark and Ireland, soaking them in the 'Dutch Zeeland' waters for some time and selling them bagged, maybe they won't be cheaper but at least they have the right 'flesh weight'
My estimate now is around 400NT/kg
I did a search for the retail price in carrefour belgium and came up with this
Carrefour start voorlopig niet met verkoop Zeeuwse mosselen
Zeeuwse mosselen zullen worden aangeboden van zodra de kwaliteit kan worden gegarandeerd
It says thet they don't start sales yet as quality is not guaranteed.
If they don't sell, than it's probably not a good idea to order them from the supplier in Holland as you can't see the product and don't know the quality ...