If you are already playing the game (Skirmish mode) the cheapest way to get Adventure Mode is either First Flight (EB) or Back To The Skies at $20.
First Flight (EB or not) would bring your current game up to the Kickstarter Edition (Awkm will have to confirm if that backdates Dustrider, but I think it will). You'd also get into the beta.
Back To The Skies would leave you current game as it is, but get you the Kickstarter Edition of Adventure Mode (presumably with Dustrider 2.0 - or is that Kickstarter Collectors Edition only?). You wouldn't have access to the beta (which seems poorly thought out given that FF EB does for the same price and includes swag).
There are a couple of sensible options missing as I see it (thinking budget conscious and non-US residents)..
- something around $30 with single-player beta and both Skirmish and Adventure Mode Kickstarter Editions, none of the swag.
- something around $50-60 with single-player alpha, beta and Adventure Mode Kickstarter Collectors Edition, none of the swag.