I work for free all the time. FOR MY FRIENDS.
If you are offering promotion in exchange for a design, you probably want to make sure your request is going to people that have heard of you. Not saying you're unknown, but it would probably be better to make the request among the viewers of your channel, rather than on a web site that isn't specifically related to music at all.
Also, if a super rough outline in MS Paint is all you want... couldn't you do it yourself? Or at least describe what you want to the guy you're paying $150?
I did it! Prince Robot IV is a success! Give me some internet love and watch the videos- Robot DJ
, Dancing Robot