You've almost read my mind with that post, EXASKRYZ. My rough plans look pretty much identical to your layout (90 Pagies in total split into three sections per level), but probably because we both used Banjo-Kazooie as a starting point!
Opening the worlds in any order is not possible, however. To access new areas of the hub, you need to unlock special moves from the levels. For example, in Banjo-Kazooie you had to go to Mumbo's Mountain to learn Talon Trot before you could make it to Treasure Trove Cove. Likewise, you had to go into Treasure Trove Cove and learn the Shock Spring Jump move to make it to Clanker's Cavern.
No problem though. If the moves are put into the first variation of a world; a player can get in with relatively few Pagies, collect the five or so Pagies that are present, learn the new move and leave for the next world.
5/3/2 Pagies per world variation seems to work well. The last variation of a level could have a boss and maybe the last "Jinjo" equivalent collectible. Of course this format can be adapted to suit specific levels (nobody wants the game to feel predictable), but it could work like this as a general rule of thumb.
I think we've explained the concept of expansion cost and Pagies to collect per expansion quite well, but the maths is too complicated to discuss properly in words alone!
Not to mention the specific numbers will likely need tweaking depending on context. E.g. if a boss appears in one level, or a certain move needs to be learned, some numbers will probably have to change. We simply cannot predict it, so we'll have to leave the totals/Pagie costs/mathematical stuff to Playtonic!Maybe the rest of the thread should be devoted to the more practical aspects as the original post talks about. E.g. do you need to leave the level to expand it? How can we stop this process being too time-consuming? So on, so forth.