Negotiating debt settlement

I stupidly cosigned my brother's lease and he got evicted The balance is now $42k. Yes $42k, not a typo.

He filed for bankruptcy. The collections already hit my credit score.
I want to ask for a settlement contingent on the collections being removed.

What is a reasonable amount to ask for? I assume something like $10k is unreasonable but even if I could get it down to $30k, I'd be relieved.