If you consult textbooks in corporate finance, managerial finance, etc., you will find the following procedure for evaluating the buy versus lease decision for a
business.* To simplify matters, assume the lease period equals the planning horizon.
1. First, using standard net present value (NPV) analysis, determine that acquiring the services of the equipment is justified; i.e., investment in the machine has a positive NPV.
2. Determine the after-tax cash flows associated with purchase of the machine over the planning horizon. These cash flows recognize the tax shield provided by depreciation.
3. Determine the after-tax cash flows associated with leasing of the machine over the planning horizon. These cash flows recognize the tax shield provided by lease payments.
4. To provide a fair comparison, adjust either the cash flows in #2 to allow for sale of the used machine at the end of the planning horizon or the cash flows in #3 to allow purchase of the used machine at the end of the planning horizon.
5. Discount the after-tax cash flows from purchasing and leasing by the after-tax cost of debt capital.
6. Select the alternative that has the lower present value.
Choosing the optimal level of debt financing for a firm would involve a discussion of optimal capital structure. As I have probably already worn out my welcome, I will pass.
Steve
* Some finer points are omitted.