I think you may be a
Rixdaler which is an alternate spelling of Ruksdaalder, a Dutch 2 1/2 guilder piece
My prefix survives above the tree line without you
Rix is a dialect variant of Rush, specifically the meaning of: any of various plants especially of the genera Juncus and Scirpus. Juncus trifidus (Highland rush) as an example is found in boreal and alpine cliffs, ridges and plateaus above treeline.
My infix makes 500 minus 9
IXD is one way to write 491 in Roman numerals, though CDXCI is the proper way.
My suffix is always a little amped up, or maybe down?
Daler - Daler Mehndi is an Indian performer/singer. In such a role he is amped up? Currently he's facing criminal charges for human trafficking, thus amped down?
Additional note:
As @z100 points out, there is also a Rix-dollar (hyphenated per Merriam-Webster). This would make the suffix "dollar" which is up or down with regard to AMP (a crypto currency)? Though I don't particularly feel strongly that "daler" is what was intended, I don't feel any better about this version mainly because of the hyphen. Now if the infix clue was 9 minus 500, then ix - d would make much more sense to me!