Artwork photography

[oriolcara]

I've been asked to photograph several pieces of artwork, mostly ceramics. I already have some experience on that but I've never done it for money (I worked in an art school for a few years and it was part of my job). I did a session with that artist for free (my intention being to be able to get some connections with other artists but it never happened). Now she has contact me again.
I'm not going to work for her anymore unless she pays for the work. My problem is that I don't know how much to charge. Should I charge for piece or should I fix a price per session (hourly)?

I would appreciate any comments.

Oriol