The way I view it is from the perspective of worst and best case scenarios. Worst case none or only one expansion is released. If only one what would be the cost of it?
To use US dollars the number that float around in the back of my mind is $34.95. This seem likely as landing on moons and all that goes with it is a huge expansion of the game and justifies that price. Landing on planets is even bigger with all those will entail, so same pricing. So if only 1, the expansion pass pays for itself basically and the added benefit of deciding do you have the money for it on release? Is it worth it, or wait and see? The regret why didn't I buy the pass when I could. Of course if none released you regret that as well, but it was money spent in the past, and like a night out, just a memory. The cheapest I envision would be $19.95 for a small expansion.
For some it is how do I convince the wife to spend more money on the game? Buy the lifetime pass and you get a lifetime wife escape clause and as some say the equal expense for her. One time charge far cheaper. Just imagine every 9-12 months telling the wife or girlfriend, there is a new expansion I must have.... Anyway you get the point, one time charge and yes a hope for at least 1 expansion, but 2 or more and it pure profit from then on out.
There is a last possibility, FD at some point may release the game for whatever will be the normal price and include all expansions up to that point free. Lets say 3 expansions, and new people get them all for free. This makes sense as not many will pay $50 for instance and know they need to spend more $100 more to get all the additional content, but come the next expansion they will have to buy it.
Anyway to me it is worth it. The money I spent on it is long gone and forgotten, from here on out if anything new is released, I will have a night out for basically 'free' just saying I bought this expansion and FD is treating me to a night out, on their dime.
Calebe