Okay, this is typical of floating point calculations.

You may be able to help the display by explicitly rounding, i.e. change the
calculation from:
   [grams_unit] * [quanity]
to:
   Round([grams_unit] * [quanity], 2)

However, the floating point error will still be present. An alternative
approach would be to use a fixed point number, such as Currency. By forcing
the calcuation results to this type, you avoid the floating point problems.
By default, Access will show the result as dollars and cents (or whatever
your currency is), butyou can circumvent that by setting the Format property
of your text box to Fixed. The calcuation would be:
   CCur(Nz(Round([grams_unit] * [quanity], 2),0))

For info on why floating point numbers cannot accurately represent real
numbers, see:
   http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems