Danny Trevathan got 4 for 24.5 with 12 guaranteed. He was an ILB for the best defense in football and was also known for his coverage ability.
That seems like a fair assessment of his value. You basically pick a brown and keep him. You can keep Zach this year and lose Preston next year, or vice versa.
The D-line depth we have with dareus, douzable, bryant, washington, and worthy makes Kyle leaving fairly likely IMO. Keeping those other guys would be cheaper as they're largely relatively young.
The only thing that doesn't count is players that you actually release from under contract. They do look at salary largely with this formula though, so despite brown and alexander being great players, they may not net us more than a 6th or 7th.