3.5% of all mining rewards will be distributed among all Secure Nodes. Though Its hard to know many Secure Nodes there will be, going by the testnet figures my guess is you can earn 5 to 8 Zencash a month for operating a Secure node.
It is exciting to the zencash devs getting things done. But I believe that there will be many more nodes than what is on the testnet. I bet there will be 5000 nodes within months of node payments moving to the mainnet. There will be even more nodes as the price increases.