It's also worth bearing in mind that a typical breakdown of how the cost of clothes is divided reads something like:
~60% retailer markup.
~ 10% brand profit (if different from retailer)
~ 10% material costs
~ 10% transport
~ 10% other (inc. Factory profit, labour costs etc).
Which means that if a dress cost £30 for normal sizes, then a plus-size dress would have to require three times as much material to properly justify an increase of £6. Maybe there's an argument to be made that transport costs etc might also be impacted (larger clothes requiring extra packaging arrangements, etc), but even so the dominance of the retailer mark-up means that you can't justify price hikes on this scale based on extra fabric usage.
Put more simply, while it's not unreasonable to expect larger clothes to cost more, it *is* unreasonable for them to cost *that* much more.