If I understand your question correctly, it's actually the opposite:
> some firms charge a flat retainer fee, while those that specialize in helping companies negotiate a ransom payment tend to charge a percentage of the total ransom amount paid.
Having a good relation with the ransomware group means that the group can ask for more knowing that the negotiation firm won't object too much and that the ransomware firm stands to gain more. Other than the victim, everybody wins.
> some firms charge a flat retainer fee, while those that specialize in helping companies negotiate a ransom payment tend to charge a percentage of the total ransom amount paid.
Having a good relation with the ransomware group means that the group can ask for more knowing that the negotiation firm won't object too much and that the ransomware firm stands to gain more. Other than the victim, everybody wins.