Rent or buy: a break-even model

rentCost[m_] := rent m
buyCost[m_] := closing + (payment + maintenance) m

breakEven = First[Solve[rentCost[m] == buyCost[m], m]]

{m -> closing/(-maintenance - payment + rent)}

D[m /. breakEven, rent]

-(closing/(-maintenance - payment + rent)^2)

assumptions = {closing -> 40000, maintenance -> 500, rent -> 2600, payment -> 1900};
monthsToBreakEven = N[m /. breakEven /. assumptions]

200.

Plot[{rentCost[m] /. assumptions, buyCost[m] /. assumptions}, {m, 0, 300}]

-Graphics-