I charge based on expected time to create - I know how long it takes me to design aspects to a map. Once given scale, dimensions and detail I quote a price based on $20 per hour modified by whom the customer is. Sometimes I charge less for non-publishers. The detail in the map is more an issue than the overall scale, as I do this in vector size is not really relavent as regards to the price. A small map with lots of detail may take longer to create than a less detailed 24 x 36, for example. The smaller map would cost more.

GP