Everybody has a different opinion on this subject, but I agree with what Tiana is saying.

I have had a few nightmares. One of the first maps I did was a commission for a start-up with no money but connections to big names in the old school D&D world. I made a map for Exposure. Now on one hand he did introduce me to those big names and I have made friends I never would have, on the other, when he finally published the adventure it had been so long he forgot who made the map for him and didn't give me credit for it.
A worse experience was for an RPG adventure Kickstarter. They contacted me just for permission to use maps that I had posted online (free to use), Then they asked how much for a few original maps, I quoted a slightly low but fair price. They agreed, but then a few maps became 5 maps, and then they wanted me to significantly rework the old maps. I ended up putting in way more work than what I had agreed to. The Kickstarter funded but didn't perform like he thought it should have and he went all over the internet blaming me & my maps, NOT Cool.