Flows, model A (1,2,3,4 years): -200, -200, -250, -400+500=+100

Flows, model B (1,2,3,4 years): -250, -250, -250, -250-50=-300

Present Value each flow, add 'em up, compare to purchase price: pick best of the 2.

Example of PV calculation, using 3rd year -250: -250 / 1.15^3