I would buy a gift that's in your regular price range for the child your child knows and then a token gift for the twin. The parents really should have written the invation as Olivia's birthday party to her friends and another one saying Adam's birthday party for his friends. But since they didn't it's one of those nutty dilemmas. Buy Adam an action figure or nice book that's reasonably priced - and get Olivia waht you'd get her for any other kind of party.
Birthday parties get expensive - around here the standard gift range is $20 - $30. As much as I hated to do it there were parties we declined if my kid wasn't a close friend. It's tough becuase for elementary school age kids birthday parties are the total social scene and they really feel like they've missed something if they don't go - particularly if the kids are in the same class. On Monday the weekend party is all the buzz at school. You have to make decisions on these things one a time and you never really know for sure if it's the best decision.
Oh well! Another parenting dilemma - what a surprize!