It kind of depends on the cartographer you choose. Some can perform faster than others and have different skill levels and skill sets. If you look around the forums or browse the galleries for a mapper that has done something in the style you like then your best bet may be to contact them directly with your guidelines. They can then in turn give you their estimate.

Some helpful information to include is how big the final product should be, Whether its for print or not, Time frame if any, stuff like that.

I usually base my price on how many hours i think it would take me to finish the original request with the information given then add a few more hours for corrections and adjustments. That's just me. Others may have another method.

Hope this helps you out in some way. Good luck