All value is subjective. The answer is, whatever someone is willing to pay.

And since when are reposts costly? There is no scarcity for reposts. They may be costly reputationally, if you are reposting content others wouldn't want to see. Outside of that, though, it costs nothing to repost except a couple seconds of your time.

Since Bitcoin is perfectly scarce, and reposts are not, it follows that the value of reposts would trend toward 0.