I've always seen rent defined as payment collected in excess of the seller's cost to make and hand over the product (not the whole product line, just the individual item). In a perfectly competitive market, price is driven down to match that cost, but the comparison with production cost is easier to apply in markets that aren't perfectly competitive (like licenses for a particular patent). Once an invention has been developed, the marginal cost (to the inventor) of someone else's use of that invention is zero. Any license fee at all that the inventor collects is in excess of what it costs the inventor for the licensee to use the invention, so the license fee is rent.