Q. Why is it cheaper to use Factom than Ethereum or Bitcoin?
It has to do with difference security assumptions. In bitcoin and ethereum, the hashing process means that blockchain packagers are known after the fact. There are time sensitivities involved in block propagation with those systems.
In factom, the blockchain assemblers are known ahead of time, so there isn't quite the sensitivity to latency of large chunks of data as the other systems.
There is another tradeoff. to validate bitcoin is correct, you need to iterate over the entire blockchain to see if there were doublespends. With documents, the corollary of a double spend is only useful in the context of those who are interested in those documents. People who are not interested in the documents don't need to verify or even download that part of the blockchain.
Since the data can be separated by the factom sub-chains, There is also no guarantee that your particular data gets stored and hosted by many nodes in the network. If the data in that sub-chain is important for you and your business counterparts, you can store and host it yourself. The entries will still work regardless of who hosts the data, since the hashes match.
(this is thinking out several years)
I kind of think of it like newspapers, where you aren't guaranteed to go down to the corner store and get last week's edition, but you can go to a library or some longer term storage system to get what you need. If there is a special edition, you can just keep it yourself, and it is self evident that the newspaper is authentic, without needing to refer to some library microfiche.