Prawn vary greatly in price, I guess a few kinds of prawn are more elitist then others so demand a price premium. I buy mine on special and normally get the cheapest.
Purchased a 1kg pack of baby Salmon today, for $9.99 . That reminds me it is still in the car thrawing out…. (edit: In freezer now 
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We probably already covered this, but do you catch noticeably less Red Cod when using Red Cod for bait. Would love to catch less of the buggers.