I have a client who wants some form of 3D ninja turtles cake but doesn't want fondant or to pay a ton. 

My idea was to make the wilton stand up bear cake, cut off the ears, and decorate/ form it into a ninja turtle. Fondant for his mask of course. Get a small sword or spear to hold in the two hands. and probably extend out the legs to kicking position. 

Is this totally nuts?

AI think using the wilton pan is a good idea, it seems like you know what you're doing.I mean you can only give it a try right?

Did you ever make this cake?  I am interested to see how it turned out for you.