Decentralized validation and ordering of transactions is expensive. In Bitcoin, it requires:
1. Miners to validate 1MB of transactions every 10 minutes
2. Users to be able to confirm that the entire chain of transactions was properly validated by the miners
Some ways to make #1 more efficient make #2 more difficult, so they are somewhat at odds with each other. For now, we're looking at a write throughput of between 1tps to 20tps for Bitcoin and Ethereum.
If you don't care about #2, then you don't need the mining model anymore, or even a blockchain. You can just put all your transactions in MySQL, Cassandra, R3 Corda or whatever and you can subsequently crank up your transaction throughput. You can get >10,000tps with MySQL.
Could you elaborate on what drives the high cost? Or what would need to change for this to viable?