I've done a few calculations based on using 1.6mm thick box and 2.5mm box. Based on using 36m of box and 1.6mm thick it comes to 42.4kg and using 2.5mm box it comes to 63.67. This is quite a large % increase in mass but in the grand scheme of things it isnt much since someone of the larger build than me could easily add double that mass.
There isnt that much of a price change. 1.6mm box is £14 per 6 m, i havent got a price for 2.5mm 25x25 but 3mm 30x30 is £20 for 6m so im guessing 2.5 25x25 would be about £17-£18 so thats £24 more maximum for the steel used.