You probably won't make as much money creating maps as doing other kinds of illustration work. I have been paid well for a few maps, but mostly create maps for $50 each, less for publishers purchasing at least 3 maps. Paizo paid me the most, but they have the money. RPG game publishers usually have limited budgets to create products from. I've done a job where I did both the cover design and a map, and I was paid twice as much for the cover art - with the same level of work involved in both. I do less and less commissions, however, mostly creating maps for my own publishing projects and may eventually only work for myself.