New scaffold boards have doubled in price here - now £37 for a 3m board. You might be able to find cheaper unbanded ones on facebook marketplaceEach upright (2400mm) and 7 brackets (for 12" shelves) is working out at around £24. You would be adding an extra 4 of these (are my sums ok?) at 12" centres. 18mm birch ply is about £110 (it seems). Not sure how much softwood would be. Interesting to see what the most cost efficient system would be.