Big assumption and evidently not the case.
Even IF all exchanges had the same fee tier schedule, you still don't know what part of the volume pays which tier fee.
To whoever is answering liquidity: liquidity and volume are two completely different things.
Liquidity means how much depth is available in order books, volume means how much value is traded.
An exchange can have smaller volumes, but high liquidity (eg Bitfinex) and vice versa.