First, all votes are worth something. Everyone likes to see that someone took the time to read and vote on a post. I have followers that never will have enough HP to contribute an upvote that generates a penny, but I still appreciate the upvotes as affirmation.
In regard to the HP, for the upvote to generate a reward for you and the author, the amount will change based on the value of Hive. At the current value of $0.35 per hive, you need around 1,200 HP to generate a $0.02 hive upvote at 100% power. You can go under your name on Peakd, select dashboard and then analytics and it will show what your vote is worth. Don't forget, an upvote of $0.02 Hive is worth 50% to you and 50% to the author. And so the amount of real USD generated is $0.02 divided by 2 times current Hive value, which makes a $0.02 Hive vote worth about $0.0035 today.
Also, if your upvote is less than $0.02 but there are other votes on the post, then you will receive some payout as long as the total of all upvotes is greater than $0.02 hive.