do i need just the core game to get trophies or do i need both expansions?  also are the expansion $5 each?

I thought that you only needed the core game but I'm not 100%

You need the expansion for, Extra Ships Optional: Survive 7 minutes in endless mode, Late Boomer: Get 15 bombs in bomber, Close encounters: Survive 45 seconds without shooting or using a bomb in survival, Shock and Awe: Destroy 10 nukes in endless, and Split Screen Co-op, get 3 minutes of continuous co-op weapon boost on Segomo.  All the others listed on the Super Stardust HD page you can get.   Yeah, the expansions are $4.99 each and there are two, so technically the whole game is $20.00.  In my opinion the game is well worth it, I'd pick it up if you're thinking about it.