The calculator app is just that. A general calculator, not a business calculator or cash register.
You can sort of get it to do what you want by changing the settings to use 2 decimal places:

But, if you don't enter a decimal with any following numbers of 1-9, then it still won't show anything but a whole number. If you do enter any decimal numbers, it will round off to two places when set to 2. But any decimal amount that adds up to zero will once again only show a whole number. Anything that results in the second decimal place being a zero will only show the first decimal. Such as 234.3, rather than the expected 234.30.
I always use PCalc. I can set that to use two decimal places, and to always show trailing zeros. Then I get a standard monetary readout all the time.



But there are lots of calculators in the App Store. Many of them free. Try some to see if any do what you want, then make that your default calculator app.