I think its just about impossible to gauge their value. Everyteam has different needs, and depending on who wants what, I don't think there is an empirical formula you can use here.
I like to think they'd be worth about a 10th rounder, but depending on how the draft is shaping up, and where people have players 'tiering' out, and the needs of the team holding the higher pick, it might be worth more, less, who knows?