The most accurate cost source is actual bids from contractors or to look at contracts from the last time the repair/replacement was performed. In most cases bids or contracts are not available so unit costs for similar work done in the same local area are used. In addition, it is helpful to talk to local vendors who have knowledge of the work and can help with a cost estimate. A third source is to use construction cost estimators such as RS Means. Many times the entire quantity of a component will not need to be replaced or repaired all at once. An example of this is not all light fixtures on a property will need to be replaced at the same time. In this instance an allowance can be developed for the component.