Most independent hoteliers I asked this question answered alike: not a clue. Which is not that strange, as we are all used to clear cost for services we use, much unlike costs of payments at typical banks.
Go ahead and check your banks batch payment statements including held back costs per transaction. One thing is for sure: you will find hugely differing costs for exactly the same reservation amount, over and over again.
It is due to the payment card that was used to pay and due to the traditionally complex, multi-tier cost structure for credit card payments. The bank that issued the used card and the credit card firm take a fair share of the transactional fees, but most of it ends up in the pockets of your bank (or acquirer, or Payment Service Provider in financial lingo). And these costs can easily differ by a multiple of 5+, dependent on which card was used.