Numerous bounties have as of now says it all in their standards and guidelines that BM can change the rules anytime so bounty hunters should realize that changes can occur anytime aswell
Anytime does not mean they (company, ICO team) can change rules after bounty is over and work is done. Imagine that you are bartender, you arrange deal with your employer for $500 per month, after a month of hard work your employer gives you $10 and say, I change rules of our deal because I can do that. You wouldn't blame yourself in this situation.
It depends on the coin/token itself. You have a right to blame the bounty manager if the amount of money is less form your expectation. I mean, you took an instance with different cases, 1 dollar will have the same value because the dollar price is not have volatility price. Different with cryptocurrency which is have a high volatility, I won't blame bounty manager if the payment for each bounty hunter is $500 and the bounty manager will send the amount of token which have the same value to $500 whatever the amount of the token.