Where does the value of Author Rewards come from?
Normally the value of author rewards comes from the dilution of existing stakeholders. New steem is created as inflation, which dilutes the stake held by everyone else holding Steem. All-else-equal, a Steem holder is being diluted about 9% per year and a Steem Power holder is being diluted about 7% per year. This dilution pays for your author rewards among other things.
When the Steem Network creates an SBD, it backs it with $1 worth of "Virtual Steem". Virtual Steem doesn't yet exist in the sense that it's not finalized, how much will actually be created is an unknown until the 'convert' contract is executed on the SBD.
You also get 50% of the nominal reward in Steem Power, which is not virtual.
Things are a bit different right now
We are in an unusual situation today where dilution is not actually the main source of value from rewards. At time of writing this the price of SBD is $5.66, when it is intended to be stable around $1. This means for every 1 SBD you have, you can sell it for $5.66 and get that much purchasing power, but you're only being backed by $1 of Virtual Steem. That's only 18%, a little precarious if you ask me.
Conclusion
If you calculate the total backing of rewards by dilution, including the Steem Power part of the reward, only 30% of the dollar value of the reward is supported by dilution.
The other 70% is purely SBD speculators.