I agree entirely, as I’ve said I will try to have a go at OSB this year but can not see any real advantages of Rose boxes over supers.
Regarding cost I buy supers and frames as seconds so pay approx £15 & 50p but have never seen any OSB sizes in a sale.
The cost for these items on the Thornes website is approx £30 and £1.70 respectively
Hardly the 10% you quote although the boxes are easy to make so I would make my own if I went for that size.
Even the cost of the foundation is significantly higher, £1.05 compared to 74p.
The need to have 20% more supers than Rose boxes mitigates these elevated prices slightly but using supersI could make up a 30” tall hive for 60% of the price of a Rose hive of the same size and as I already run standard supers on my hives I can use my existing ones.
As I’ve said before I like the idea of a OSB but I’m yet to be convinced that Tim Rowe’s choice of size was based on anything other than it was halfway between a brood box and a super.