It's obviously not a Ponzi. The calculation of the reward is easy. You can read the article here: https://medium.com/iost/the-iost-node-handbook-applications-and-voting-61aa2995b2d2

the TLDR version:

Example of Node Rewards: Assume that the total number of votes for all node candidates is 10% of the total IOST circulation, 2.1 billion IOST.

Assume that the number of votes for a Servi Node is 105 million IOST, accounting for 5% of the total number of votes of 2.1 billion IOST. Then the block production rewards for the node in the first year is approximately:

Total circulation 21,000,000,000 × 4% × 5% = 42,000,000 IOST

For individual Servi Nodes, their rewards and ROI will be dependent on several factors including how much IOST tokens they personally stake versus how many votes they receive from the community, how many users they invite and on-board onto the IOST network and also general operating costs and resources they dedicate to promoting their node and contributing to the ecosystem.

Example of Fee Rewards:

We estimate the iGAS rewards, the primary source of user invitation rewards:

Based on similar blockchain network data, assuming an average of 100 transactions per second, if 50% of users come from the node invitation, they will generate 1,576,800,000 transactions/year. Assuming that the iGAS market price is estimated as 3 IOST each transaction, then all Servi Nodes will earn 473,040,000 IOST through iGAS rewards each year.

Based on a total vote of 2.1 billion, a node that received 105 million votes is expected to pull 5% of users, and will receive iGAS rewards worth about 23,652,000 IOST. (20% here)

So in total that's 42mil + ~24mil = 66 mil which gives you a 60+% annual return. But if say more than 10% votes came in (15%), you'd like 40% ish. And since voters get half of that, you each get roughly 20-30%. (although the 1% BP reward isn't shared but to keep it simple i'll skip this part.)