How much Steem Power makes an upvote worth $1.00?

Variable exchange rate: Internally upvotes are based on STEEM, not dollars. So your account's upvote dollar value will go up (or down) based on the market price of STEEM.
Competition for limited reward pool: The reward pool is rewarded on a no-reserve basis, meaning that if only one person upvotes posts, their upvotes will control the entire reward pool; the total of their pending upvotes at any time would be worth about $200,000 at current market prices. Stated another way: If Alice's account was idle and then becomes active, the rewards for Alice's upvotes come from slightly reducing the rewards for everyone else's upvotes. If Bob's account was active and then goes idle, the rewards that used to go to Bob's upvotes instead slightly increase the rewards for everyone else's upvotes.
Payouts are nonlinear: As a post accumulates upvotes, its rewards per upvoting SP increase. So if your account adds $1.00 to a new post, it will add a lot more than $1.00 to a post in the $5000+ range.
Soft rate limiting: Each account has voting power which decreases on each upvote and regenerates at a constant rate, so if you vote a lot, each individual vote will be worth less.

For reference, this account's upvotes are worth about $0.20 - $0.30 for posts without any upvotes.