What I'm best at is RPG writing (settings, adventures, characters, historical resources, GMing advice and more), and I no longer set a rate for that. During my freelance days (1991-2001, approximately), I did RPG writing for a wide range of rates (from 4 to 15 cents/word). For most of my independent days (2001-2016, approximately) I kept my rate locked at 15 mainly to keep people from bothering me, and then still dipped a toe in to keep ties with certain friends and colleagues.
These days, my rate is just "impress me with how much fun the project is," combined with "offer me something Cumberland can't." Perks are more interesting than pennies. A great editor/writer relationship exceeds the value of any currency. A fascinating world to play around in beats them all.
I have no modesty on this point: if you have me on your book, your book will be better. But, be amazing, or get lost.
My editorial rate (full-project only) is "half of what the writer gets." My production, typography and/or indexing rate is $15/hour.
For a graphic example of that "per-structure" rule: the Scott's Landing map would be covered by the standard $5 rate, since it's not insanely packed with buildings, walls, or boats (it would be a $400 commission), while the Trostig map has just over 2,000 rooftops and a handful of boaty-boats, so there the "per-structure" rate would dominate (it would be a $550 commission).
These rates are for pen-and-paper RPG publishers only. My non-RPG graphic-designer rate is $75/hour. My non-RPG writing rate is "no." If you've got more questions or want to check my availability and timetables, my inbox is always open.