Your calculation is correct. Its kind of a bad time to get Sins right now, as the option that is the best for your money won't allow you to immediately play the game.

So unless they decide to allow pre-orders to automatically come with trinity, you'll have to wait until the Rebellion beta to be able to play the game in any form. Though of course that will save you $30.

Maybe they should offer to sell Rebellion with current access to Trinity for $50.  That way, people would have to pay a $10 premium to be able to play the Trinity content now.  It would essentially be like selling Trinity for $40 and Rebellion for $10, but Stardock and Ironclad would rake in $10 more than if someone waited for Rebellion.

I don't think Stardock and Ironclad are going to try to nickel and dime people like that.  The early adopters, at least people who purchased the games prior to the Rebellion announcement, are not going to get screwed because they have been able to enjoy the game all this time.  At some point, years from now, the price of Rebellion will probably drop to $10, but who wants to wait that long to play a game?

I do wish that the discount for Diplomacy/Trinity owners were a little greater, say $15 or $20 since they would essentially be repurchasing Trinity, but if Rebellion is really good and revitalizes online multiplayer I won't complain.  There might be some logic to charging ecstatic fans of the game a little more money--they have already derived more entertainment value than what they originally paid for the game.  I know that I have, and I'm into it for $60 now.

Like what the others said.  If you want to play the game now, buy it.  If you can wait and don't want to play it now, then don't buy it.

If you want to buy it now but are having a hard time deciding if you should or not, let me give you something to think about if you have not already:

If you buy the game now, and you play it say, 6 hours/week (which is not a lot) you will get more than 200 hours worth of entertainment value before Rebellion is released (I don't know the exact date of it's release, but lets say its released in December).  That means you are paying less than 15 cents per hour of entertainment.  The more you play the game, the more value you for your money.

Compare that to going to the movies.  Lets say you go to the movies once a month and you pay $10 for your movie ticket (perhaps more or less depending upon your city, but in Los Angeles, you pay more than that).  Lets say each movie you see is 2 hours.  That means between now and Dec, you will pay $80 to your local theater, not including the cost of gas to drive there or any food you buy (8 movies, apr - nov).  That's 16 hours of entertainment which cost you $5/hour, and you can't watch the movie again.

I'm sure you already know this, but perhaps you didn't break it down like this.  Games are a great value for the amount of entertainment they provide...you just have to be careful to not allow them to take up so much of your time that it prevents you from doing other things you want to do in life.

I say, if are thinking a lot about it and will actually play it a lot, get it now and when rebellion comes out, then get it too.  Yes you will have paid a little more to the company, but these developers are giving you a lot of entertainment in exchange.  A lot more than you get from your local movie theater.  If Rebellion was coming out in the next month or 2, I would say wait, but we have over 6 months left so you have plenty of time to play before Rebellion hits shelves.