I wish there was an easy answer to that Q - there are posts here (if you search) on how the rewards are calculated but the $80 is an average of the value of steem over the past 3 days I think. It's then split into steem and sbd. The benefit though is that the sbd is treated as $1 when they're actually worth closer to $3 at the moment. You can find latest values at coinmarketcap.com. I wish it was easier to explain!