Matt, does the RSP help at all for redraft leagues?
That being said, how many rookies played a big role in redraft last year? Enough to be worth $20? That's up to you.
In redraft, I took his top ranked rookies (based on the post-draft RSP), a couple in mid-rounds (Lacy, Gio) and a couple very late (Corderelle, Keenan Allen). It helped me quite a bit.