Even though at resource rate may be set for cost per day or cost per hour, internally Project calculates all time related data to the nearest minute. For example, a resource rate of $1/hr will show a cost of $.016 (rounded to $0.02) for a task with a duration of 1 minute.
If you don't want to use the "cost per use" then I suggest you set up your tool costs as cost type resources. In this example the bomag is a cost type resource so no rate is given. When the bomag is assigned to a task, the rate is set in the Cost column of the Assign Resource window. This cost will be fixed regardless of the task's duration.