We can assign an abstract value to Seattle moving from the 40th overall pick or we can try and see exactly what players it might cost us. I would rather do the latter. Seattle is only truly set at two positions, linebacker and center, so we can be pretty sure it would have targeted the best available talent at 40. 40 is widely considered one of the most valuable picks in the draft, because it is very close in quality to latter first round picks, but cheaper. 60 is similarly cheap, but gives a team less flexibility when it comes to matching scheme and matching need.
Predicting the draft is difficult because one is attempting to predict a sequence. It is a lot easier to predict what picks will be drafted in what round, rather than who will draft them and at exactly what pick. Basically, by this point in the draft process, we have a good idea about a player's value.
40 should be populated by players that are valued as late-first round picks and early to mid-second round picks. I figure late is broadly the final third, since we describe it as late, mid and early, and so the range of players that could reach the 40 pick, but almost certainly will not reach the 61 pick are players ranked ~21-43. Who are these players? Well, let's look at one draft sites to form an educated guess.
The following is a list of players rated from 23 to 40 on the site NFL Draft Scout. If anyone has another site they respect and would like to suggest, feel free. I grouped them by position.
Of the above, I would say it's highly unlikely the Seahawks target Anthony Davis, Brandon Spikes, Sergio Kindle or Daryl Washington. Everyone else fills a need and theoretically fits. Pouncey is a center but could play guard. It is possible one of the above slips to 61, but unlikely. It is likely that even more players will be gone, but that is much harder to predict. The above players should no longer factor into the Seahawks plans.