With radio buttons or a single check box (NOT multiple checkboxes) this will be fairly straight forward.
Set up a related table with one record for each radio button value or your single checkbox value.
Standard (Standard charge amount here)
Then link your tables by the radio button field:
YourCurrentTable::RadioButtonField = LookUpTable::Category
Then a looked up value auto enter field option will copy teh value from this table when "standard" is selected in the radio button field. If the user clicks "custom" no value is looked up unless you also add a record to the lookup table with "Custom" in the category field (but presumably, custom jobs have different amounts for each job.)