$100 per year expense. Of course, the time value of money is one of many factors affecting the comparison of one-time and ongoing costs. Other factors are how much money we have on hand, how much we can borrow, and how quickly loans have to be repaid. The typical scenario for comparing up-front and ongoing costs is the decision to pur- chase equipment or to lease it. In addition to the time-value issues of money, we also have the time value of equipment and the element of risk associated with that. The ob- vious risk is that something will break down and leave the owner responsible, but most of our wireless telephone equipment is sophisticated enough to have service contracts and maintenance schedules. The more subtle risk is that equipment will become obso- lete, that a year or two from now there will be much less expensive alternatives that provide the same or better service. An example of an own-versus-lease decision is backhaul transport from a base sta- tion to the mobile switching center (MSC). We can pay the monthly tariff for the re- quired number of DS-1 or E-1 links for the packet pipe, or we can purchase the equip- ment for a microwave link between the base station and the MSC. Of course, the cost and feasibility of the microwave link depend on the height of the base station and the terrain between the base station and the MSC. Once we determine that we can use a microwave link, and once we determine the up-front cost of the microwave equipment required, we can compare that one-time cost with the monthly tariff of leasing the DS-1 or E-1 facilities over time. However, if DS-1 or E-1 lease rates drop, we may re- gret our investment in the microwave link. 46.1.2 Equipment The normal price of a piece of equipment is the price per unit item. We expect each am- plifier to cost some amount, and we expect 20 amplifiers to cost twice as much as 10. While evaluating the cost per unit of a piece of equipment, we should consider how modular it is. A machine that does twice as much for less than twice the price ma