When you cast an upvote, you do not spend any HIVE from your account. In other words, you still have exactly the same amount of HIVE and Hive Power as before you cast an upvote.

Therefore, an upvote cannot be presented as an expense.

At the same time, whatever the value of your upvote is, you're going to earn about half of that as a curation reward. And that income, you must include in your tax return.