Yes, as I said.... the cost depends entirely on the volume of construction required and that's dependent on what capacity can be reused..... but their figures (and similar at many other big clubs) clearly show that generally it is far cheaper for larger stadia to redevelop.
In many ways GP is a far better candidate for that treatment than Anfield. All the lower tiers can be kept and/or reprofiled, and larger new upper tiers built to add capacity get rid of all obstructed views and increase concourse space etc. So that means upto 31k is reuseable (27k if the top Balcony was to be replaced) only the upper Gwladys st and upper Bullens would need to be replaced. This would also be much cheaper per seat because you don't have to go up 4 floors before you've added any new capacity. The roof cost is generally proportional to the square of its depth.... and this would be reduced because you can have overlapping tiers due to the existing tiers being much lower than those at Anfield, where there is little to no overlaps. Getting from 31k to 53k (ie. 22k new seats) is nearly always going to be much cheaper than building 53k from scratch. Of course, that's not factoring in differences or benefits in terms location etc.
You may be right in your cost predictions, but that's very much dependent on what they can save from the existing stands on the 2 other stands (if anything). However, the two new stands have come in at an average of just £100m each (full project costs) for far greater volumes of construction than any side at BMD.... these stands are as big as Wembley's, with 92 and 86 rows. Repeated on all four sides would have them upto 75-80k. For them to build that from scratch would cost well over £1bn. So even at your figures that still a massive saving and probably even less than our costs for a far larger stadium. That's the main reason we are not comparing apples with apples. If they'd just needed what we are going for, they've achieved it for relative buttons by (the admittedly crude) comparison. The benefits for them of course are obvious and multiple.