Regarding the regulation numbers, if we are to buy the whole shebang again every time they make changes which is every couple of years then I don't see the point of keeping the same edn number, they may as well instead of having 17th edn regs including corrigendum, amd 1, 2 and 3 just make it the 18th edn.
That's obviously true. As far as users are concerned, it's essentially a semantic question as to whether one talks about 17th ed., 17th ed. + Amd1, 17th ed. +Amd2 and 17th ed. + Amd3 or 17th, 18th, 19th and 20th. I presume that, in the eyes of IET/BSI/JPEL, a 'new edition' is much more of a 'complete re-write' than 'an amendment' represents, but that's clearly an essentially arbitrary decision which, as I said, is really of no relevance to users who have to buy the whole thing, whatever it's called, every time.
However, as I said before, particularly in the case of the current Amd3, so many pages are affected that attempting to 'paste in' the amendments would not really be practical. As others have said, that would only really be practical with a loose-leaf format and, even then, if a high proportion of pages were involved, the cost would probably not be that much less than a whole new 'book' - not to mention uncertainties as to whether or not all the changed pages had, in fact, been substituted.
As for the actual money (for BS7671, OSG, GN3 etc.), we're probably talking about an amount, once every couple of years, equivalent to an electrician filling up his van with fuel a couple of times.
Kind Regards, John