Yeah... I am not in the security field and I don't know what is a fair price for this. Obviously he did some work and probably in the range of 4 to 20 hours without seeing the report and being an expert on the white hat stuff.

It sounds like he was insulted by $10 and CEO response. Given that DAO is not rich and we don't know how much time he spent a fair negotiation starting point would be $1,000 to $2,000