Plan the whole line before you research the pump. On its own it does nothing: it needs power to run, a hose to send the water somewhere and, on the coast, a powered water purifier to make sea water usable. The hose is a default craft everyone knows. The purifier is another Level 2 blueprint, so a coastal base learns two where an inland one learns one.
The bill itself is easy. Metal fragments and wood come from a single farm run. The lone gear is the line that waits, since gears are a Level 3 craft and loot until then.
Power is the real cost. A pump on a small generator burns fuel to water your crops, so move it to panels or a turbine once you have them.
