There are many things that may effect the cost of producing illustrations. The size of the final image, the amount of detail needed, the reference material supplied, if any, the amount of images involved and the requested style required for the finished illustrations, can all have an influence on cost and as such this can only be used as a general guide for producing a similar, simple image, as the eye image here. The costs shown are what I would have charged had I been commissioned to illustrate this image in these certain styles.
The styles are just a general guide as well as there are endless possibilities. Sometimes the style will dictate the level of detail and amount of work involved and so dictate the general pricing level.
If you need more accurate costs please get in touch. I can supply a quote at anytime or try and work within your budget if need be.