Gas, in plain language

Ethereum and the chains modelled on it charge for computation rather than for size. The unit is gas.

The two numbers

• Gas used. How much work your transaction takes. Sending ether is cheap. Moving a token costs several times more because it runs contract code. Interacting with a complex contract costs more again.
• Gas price. What you pay per unit, set by demand. This is the part that spikes.

The fee is the two multiplied together, which is why the same transfer costs wildly different amounts at different times.

Practical consequences

• A token transfer always costs more than a plain transfer on the same chain.
• A failed transaction still costs gas. The work was done even though it did not succeed.
• Waiting for a quiet period genuinely saves money.

On a chain where a transfer costs more than the amount being transferred, consider a cheaper network for that asset.