Vehicle and business costs
Total cost is the down payment plus every instalment; break-even is fixed costs divided by the profit on each item.
A worked example
With fixed costs of $5 400, a selling price of $22 and $7 of materials per item, each item clears $15, so break-even is 5 400 / 15 = 360 items.
The mistake people actually make
A low monthly payment is not a low total cost. Multiply by the number of payments before comparing options. Depreciation compounds, so multiply by (1 - rate) once for each year rather than subtracting the whole percentage at once.