Some relevant costs to consider may include:

Upfront purchase or investment costs

Ongoing operational costs $($labor$,$ materials, utilities etc.$)$

Expected revenues or cost savings from the purchase

Irrelevant costs refer to those not directly tied to the purchase options being compared. For example, fixed overhead costs that will be incurred regardless of the decision. Including these could unfairly influence the results.

If irrelevant costs were included, it could potentially impact future profits in a negative way. The company may end up choosing an option that looks best on paper but does not maximize the true financial benefits once implemented. Resources could be allocated to something with a lower real return.