I've made 2 and I think I'm going to make more where I make a set. I have a black/rainbow dragon with a tiny rainbow egg in its tail and a large rainbow egg with it. I also have a little purple dragon with 3 purple eggs and 3 big eggs to go with him. I think if I sell them as a set rather than individually I can save people shipping costs.
Maybe 15 minutes each, some less...it really depends on the colors and shapes of what I'm drawing. The frog thumbnails for example took closer to 15-20 minutes, whereas something like the panda would take me closer to 5 min. I charge $20/hr for all the art/web work I do anyway, so that's where 5 for $25 comes from.