The Seahawks thought Michael was worth a 2nd when they drafted him two years ago. Unless he was a failed pick, then they probably still value him as a 2nd rounder. I don't see the Cowboys giving up and 2nd for him and I don't know why the Seahawks would take less than a 2nd unless they think he is a failure. If he is a failure then why would the Cowboys want him?
The other backup, Turbin, is in the final year of his contract.
When looking for a player to trade for, there needs to be a reason the other team would want to trade the player. If the other team does not have a specific reason, then you'll always have to give up too much in a trade. I just don't see a significant reason that Seattle would want to trade Michael.
Some possible reasons a team would trade a player without requiring the Cowboy to overpay:
1. The player is in the final year of his contract.
2. The team has too many talented players at the position. Maybe they just drafted one and have a starter and the guy in the middle is expendable.
3. A Super Star player has issues and a huge contract.
4. The player is too expensive for a rebuilding team.