I found this cost formula:
As you can see, the larger the transaction, the more resources are needed to store and process it.
So, my question is: Will Cardano's transaction fees be expensive(similar to ETH gas) in the future?
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My answer is no. And there are two reasons.
Cardano is a PoS protocol. Therefore, users don't pay for electricity cost. In a PoW system, miners need to solve computational questions to be able to produce blocks which consumes electricity. This price is covered by users when users make transactions.
Cardano will have more TPS compares to Ethereum currently which also contributes to a lower transaction fee.
However, on-chain solutions have limitations. Blockchains need to implement off-chain solutions to increase TPS and lower transaction to be able to make microtransactions. Cardano has given their solution which is called Ouroboros Hydra.
The transaction fees can be changed if required "to react and adapt to changes in transaction volume, hardware prices and ADA valuations".
They are not being changed for the foreseeable future. In the longer term these can be changed but will be unlikely to change frequently. A major benefit of the ADA approach is that it gives a high degree of predictability to transactions. Each change would require proposal, planning, agreement and a hard-fork event.
Given the goals of Cardano and the likely increase in the value of ADA over time, I believe the cost in ADA is more likely to decrease than increase. If Cardano is to achieve its goal of banking the unbanked the cost will probably go down in real terms too.
Eth fees are high partly to prioritize transactions and mitigate Eth's performance bottleneck for those that can afford it. Cardano has no way to pay for transaction priority.