I've had to do that with a recent Data Warehouse project.
Budget Plan A budget had a pair of exactly what we want and need servers, outfit appropriately, with some room for growth.
Budget Plan B had costs of working with existing hardware with some upgrades (about 2/3 cost of Plan A).
Budget Plan C had working minimally with what we have, which will only carry so much for so long, and relies on a lot of luck and a damage control plan... and certain assertions that it will only carry a certain load.
Such is life.