Yeah, though I do commissions roughly for $20 per hour, I estimate the cost based on size, detail and the kind of map that needs creating. World maps, continent/regional maps, urban/town maps and encounter scale maps are all different kinds of animals. Some maps like world maps and urban maps I know will take me much longer than other maps, so the price is steeper for that. In the end, as stated above, its speed at creating the maps that make a higher charge per hour feasible. I've never made more than $150 for a map, usually a third of that is closer to the norm. I have given bulk map deals where the client was seeking 8 or 10 maps, so I might cut 30% off my regular single maps fee to accomodate earning a larger fee in total for the difference.
Some maps, many photorealistic encounter scale maps I can create in an hour or less, whereas a city map might take me 8 or more. But, of course, I am a very fast cartographer - as anyone here can tell you.
One thing though, early on when I was 'slower' ( I was never slow), I guesstimated on what my time to create would be if I were faster, so in the beginning I made less per hour because I took longer to do the work, but I didn't charge that difference to the client. What I thought should take me 4 hour to do, might have taken me 8, but I only charged for 4 hours. Now I am almost right on in my quotes for time, as I have a good idea looking at any map, how many hours it takes me.
Just to comment on Djekspek's point, as a cartographer I only charge a fee to be paid upon completion of work, not based on some ficticious royalty based on future sales which may never come. RPG pubications aren't guaranteed money makers, and if paid a small royalty, it may be a year or more before you ever earn what a straight paid commission will get you instead. Royalties are for writers, not cartographers - I would never accept a royalty for cartography.
In a way, my publications that I am involved in Kaidan, etc. is my way of breaking out of being strictly a commission paid cartographer, as I have aspirations for more involvement in RPG games than just maps. Although my income from maps are now relegated to sales performance, and not immediate commission payment - because I am a publisher, the income earned more than makes up for the difference of getting paid faster through commission work.
Just some thoughts..
GP