On the HK end, prior to the bubble, you can do home quarantine, so no cost for quarantine.
Also I think SIN reduced the amount of time people originating from HK need to quarantine to 7 days.
Maybe you can opt for low cost version and refuse to the take the tests and just quarantine on either end... But then unless you have a home on both ends, you may end up paying more for hotel on one side than the 4 tests..