Bounties are always awarded after the bug is disclosed[1].
We constantly[2] upgrade the bounties whenever we feel like we should be paying more, and we will continue to do so. We also increase the rewards from the amounts in the price list if we think they result in a higher impact than what the reporter originally suspected.
We aren't actually trying to out-pay the black market. Overall, our goal is to reward the security community for their time and help for their security research, since we both have the same goal in common of keeping all of us safe (either Google services, or open source/popular software[3]).