The starter islands tend to be the recommended areas for farming mats, if they aren't overpopulated (there's either more or they respawn faster or something).
Max level writs do have some special rewards:
Alchemy writs have a chance to get you survey maps to find get more of the common mats, and can give nirnroot (although they sometimes ask for nirnroot. But they lowered the odds so it doesn't happen as much.)
Enchanting writs have a chance to give Kutas (but only ever use Tas).
Provisioning writs have a chance at some of the higher-level recipes, although purple ones are still pretty rare. But, it also has a chance to give you the different Psijic Ambrosia fragments.
Blacksmithing/Clothing/Woodworking writs can drop survey maps to get more of the material, and can drop glass motif fragments.
All of the writs (at top-tier) have a chance to drop a master writ for you to complete.
Not sure how much is good to charge someone. I've only ever crafted for guildmates, and I only asked them to provide the rarer mats.
Thank you @DiteHart and @puffytheslayer .
The question regarding charging for crafting is when I have to put in all of the materials. I avoid charging too if the materials, or at least the most expensive ones, are handed to me.