let's see... to answer/state in order of seen.
1 year, default settings is the "base" price. password protection, alterations to settings, each additional year, etc. all come at an additional token fee. additional years (likely) are a flat fee actually, not a substantial multiplier. the two real impacts on the server that necessitate the fee for additional length are hard drive space (it does take up some, but we have substantial amount at the moment. comparative to what a single world takes up), and processing time in the hourly script. neither are immensely impacted by the length of the game, so there's no need for exponential charges in that arena. The longest possible game at the moment is from the first aircraft in the database (1919 or 1920, i forget which offhand), to the last piece of fuel data in the database at the time (presently 2007).
If you buy it and it's password protected, only those who you give the password to (and/or those who crack or otherwise snag the password) can join. Stephen and myself are also likely to be allowed access to any world, as airlines or otherwise, regardless of passwords. If you don't password protect it, yes. anyone can join.
Private worlds are one-use worlds, just like public worlds. plus side, we can preserve records for you as to ranks, routes, and other things. down side (i suppose), yoif you wish to create another private world, it'll cost you tokens. As much as I'm sticking to my policy that no matter how "fancy" a private world can get, you will always be able to (given enough time) create any world for free.
Joining a private world is a flat 30 tokens, the only difference between private and public worlds as far as token fees go is (and should be, provided it isn't bugged and hiding on me) that if you are NOT in any worlds and have LESS THAN 30 tokens, you're allowed into any public world. you ALWAYS need 30 tokens to join a private world, no exceptions.
(sorry about the out of order-ness of this one): Half year resets... interesting, however the current code cannot fully support it. additionally, how would you (theoretically) regulate which planes, routes, etc are wiped out? half by what measure (value, quantity...)? please us a suggestion thread for the response to this question, and provide any additional detail you can. i'll think about it, and see how it ranks on my present list of "feature" priorities, even if I love the idea it's gotta generally have more merit then other projects i need to do.