Not sure on how much, but consider paying for what it takes to do it correctly, as it would be real bad to have to redo or do additional work down the road. If the stringers are bad, there is likely to be additional parts that need replacing. Sorry to be the bearer of bad news.
I spent nearly 7K to redo my stringers, transom, bulkhead, motor bunks, coupler etc., the whole works. That money spent was painful, but it is a real piece of mind and now my boat is totally solid and basically brand new.