We've had this exact discussion at least once before and the miserable sods who whinge that Ashley is somehow undercutting the club eventually accept that Sports Direct is a completely separate entity to NUFC and trade in a completely different market.
Up until the point NUFC limit the supply and distribution of their kit to 3P retailers, then they're all going to be charging significantly less than the club shop.
The club sell to a different market than the regular sports retailers, they can afford to whack a price premium on the cost of their shirts.