As you work through the problem, one aspect you will confront is this: "Is the problem adequately specific? Are there enough constraints to determine if a given 'solution' is appropriate?" Anticipated service life of the mod(s) is an example, in which case you could calculate a break-even point where the mod has its initial cost covered and begins to pay off.
OTOH the problem can be OVERSPECIFIED - one constraint conflicting with another.
In either case, point this out to the instructor - backed up of course with an example or two.