Are you specifying it with a decimal point where you define the kind? The “Widget Enterprises” example in the Recipe Book page that HanonO linked has:
A monetary value is a kind of value. $1.99 specifies a monetary value with parts dollars and cents.
And at the end of 15.8 in the Inform manual it says that numeric kinds default to whole-number values but that if you include a decimal point in the unit specification you will get real numbers instead.
Just as a general programming suggestion not specifically related with Inform: Perhaps it can help to work with cents as unit, so you will have 250 cents, not 2.50 pounds/dollars. When you print them on screen you can convert them easily to full unit dividing the amount by 100.
That way you will avoid working with floating point arithmetic, at least if your operations are simply additions and subtractions.
As an evidence of the problem with that, you can open the console in your Google Chrome browser, for example, using CTRL + SHIFT + J and then write 1.1 + 1.3 to get an amazing result of 2.4000000000000004
If you sum 110 + 130 and then divide by 100 you will get a fantastic 2.4