It will absolutely be cheaper to buy the combo package.
Here's my non-official breakdown:
$350 - gets you an eagle + mouthpiece.
This is cheaper than the eagle will retail for
$500 gets you an eagle + limited edition hydratube.
This is cheaper than the eagle + a regular hydratube will retail for (my guess is its roughly what the eagle itself will retail for, but that's jut my guess)
You are effectively paying $150 (+ the cost of the mouthpiece I guess) for a limited diction hydratube.
The regular hydratubes currently go for $220ish (I think).
My guess for these limited edition ones is that they'd go for $300-$350, but that's wild speculation on my part.
If you're strapped for cash, there's no reason you can't just get the $350 deal, and a regular hydratube later (they aren't gonna run out of the regular hydratubes, I have NFI how many of the limited edition swagger circ-turbines they'll have), but it'll cost you more in the long run than the $500 deal, and you'd end up with a lower value hydratube.
Either way, my guess as the the most cost effective options would be as follows:
Most cost effective - get the $500 package now.
It's the cheapest option, and gives you the more valuable hydratube.
Middle cost effectiveness - get the $350 deal now, and a hydratube separately later.
Will cost a bit more, but has less up front cost
(May end up being an equal cost to the next option
Least cost effective - purchase the eagle + hydratube once they've officially launched
Lowest up front cost, but highest overall cost.
Also has the disadvantage of not helping out the funding campaign, and there may be a wait list at launch (funding campaign purchases are front of the line when the eagle launches)
Hope that helps
Edit:
Just noticed you're looking to upgrade, not just get a new eagle.
It might work out cheaper to do the $250 deal now, and get a regular hydratube later, but you'd be endin up with a regular hydratube (e.g. Swagger circ), not a limited edition hydratube (I.e. swagger circ to turbine)