This is a two step process -- and you'll raise your eyebrows on the second step (but it has to be done).
1. Customize the "Cost1" field (I assume that's the field where you put your maintenance cost)
1a. RightMouseButton on the column header
1b. In the menu popup select "Customize"
1c. In the "Customize fields" popup look for "Calculation for task and group summary rows"
1d. Click on the "rollup" radio button
1e. In the dropdown next to it, choose "Sum"
1f. Click OK
2. Enter maintenance costs
2a. In each task, enter the maintenance cost
2b. When you are finished press F9
F9 forces a recalculation of all sorts of fields.
_Generally_ these calculated field values are recalculated automatically (assuming you have chosen Automatic Calculation in Tools > Options > Calculation-tab) but, it appears, Cost1, Cost2, etc., are not *always* in that group so you have to press <F9>. You will see summary task Cost1 getting recalculated when you do things like changing the indent level of tasks and on other occasions. But if you've merely been entering data in the Cost1 fields then the summary task Cost1 recalculation is not performed.