Its a hard one mate, depending where you're from, most likely you're not covered by any Consumer Protection laws, and with an amount of $650, it may not be worth disputing it as court cost>recovery.
Best advice, be persistent, hit the social media, tagging poloniex and coinbase asking whats happening, and most important of all be polite. No one likes an asshole.
Keep us up to date mate, Good luck
RE: Any tips on how to get my money back from Poloniex??