Marginal revenue can be defined as the increase in revenue from one additional item sold minus total price reductions on all other units. In this case, that would be $31 minus $14 (one dollar price reduction multiplied by 14 units), or $31 - $14 = $17.
However, if 14 units have already been sold at $32, the marginal revenue would be $31 (from selling one additional unit at $31). Presumably, if the next production run were at the higher amount of 15 units, all would have to be sold at the lower price (which would subtract $14 from the additional $31 gained from the additional unit sold), as the old customers would likely find out about the lower price the new customer got.
In a monopoly (where one producer can significantly affect the market price), micro-economic theory assumes that the market is already saturated at the current market price. That isn’t necessarily the case; theoretically, the monopoly producer might be charging less than could be obtained, due to unmet or potential demand.
Demand could be increased by improvements in quality and letting potential customers know of the improvements. Or, by an increase in the incomes of customers (more might then be able to afford the item).
In a competitive market, unless all producers used identical capital equipment and methods, units of different producers would be unique, and thus price comparisons would be difficult to evaluate.