When I joined, I bought just under 2BTC worth of Steem power, which was $1100 dollars of account value. Then the price of steem dropped, but my SP never did. Initially my upvote was worth at least $0.01, but if the value of the post is already higher then my vote seems to be worth more..... however now that my account value has dropped my votes are now worth $0.008 or something like that.

I'd say its probably around 400SP for $0.01 upvote power.