This is all that matters as far as I'm concerned, if the GA for your flight got all the volunteers they needed for your flight for $500, what any other person got for any other flight is meaningless. If $500 was not enough for you, you should not have accepted it. They are not obligated to give you whatever you ask for.

Honestly, your whole post comes off as arrogant and entitled to me. GA's do not have the luxury of time to hold complex negotiations with passengers. You are certainly entitled to try and negotiate with them, but if they are unwilling to do so, I don't see anything unreasonable about that. If I was a gate agent I would have probably just thanked you for volunteering and said I'd call you back up if you were needed, then put your name on the bottom of the list.

As for those who think the offer you make at check-in is meaningless, I don't understand that. The only time I've been bumped in recent years I had put in an offer of 'just' $300, knowing that if I took a bump from the early evening flight I was already on I wasn't going to accept more than a few hour delay (i.e., I wasn't overnighting anywhere). At the gate they never asked for volunteers, but they called me up, they gave me the $300, and gave me a boarding pass for the flight at the next gate over that was already boarding, the connecting flight they put me on was scheduled (and did in fact) arrive earlier than my originally scheduled flights. I've seen several claims that the bidding process is for 'data only', but in my experience that is not the case.