It seems to me the kind of deal using Miller that would get a first would really be a pick upgrade swap. Like Miller with 50% retained to swap our 3rd or 4th for a late round 1st from a playoff team. If you are adding to pick total then a 1st seems like a stretch.
Why would a trading partner agree to that? As opposed to simply trading a lower-round (mid-round) pick? If the market rate is, e.g., a 3rd-rounder, and a trade partner lacks a 3rd rounder, I'd see them either:
a) offering a 2nd rounder as overpayment
b) offering a 4th plus an additional 5th, 6th, or 7th this year.