Real good comparables are Perry and Girioux, both top-5-at-position guys that recently re-upped post lockout. Their average per is ~$8.4m. Add 5% for expected average inflation over of the deal and you have just under $9m per as a comp. But there are many reasons to believe they will take less: "goldilocks" big market to make nice side cash while not living in a fishbowl, already have a big contract in the bank, great team outlook with perhaps the the league's' best run franchise.
I'm not sure how much that takes the number down, and Stan doesn't low-ball guys that perform, but I think it's something. i''ll say $8.5m per. Worst case cap-wise, the hawks trade Seabrook for a haul or to move up in the draft to get a future top pair guy (which might be a smart move in any case). He'd be missed but it wouldn't change the hawks status as serious contenders. And better to get a nice return for 2nd tier core guy than to lose a franchise player for nothing. .