A head and torso pencil portrait such as this would cost £175. This is the starting point for pencil prices. Paintings start at £400.
For both, the price would depend on the complexity of the commission. I would individually price a piece before before starting after discussing the requirements.
The average time it takes me to complete a painting is a week. This will vary depending on whether I am working direct from life reference or adding design details. The former can be seen here. An example of the latter is both the wings and mirror surround in this picture. A painting such as this is closer to two weeks work. Designing and painting the mirror surround alone took several days. This picture would be priced at £1,000.
Another factor on price is whether I am supplied with a reference pictures or take my own. I can work with both but it would depend on the quality of the supplied photo if that is what I am using. I can advise whether a picture is usable.