I would start by saying that the work sounds interesting but that you were looking for a higher hourly rate than what they are offering. Outline what you have been getting previously and let them know what industry standard is here. They may have no idea they are offering too low or they may be deliberately trying to get you cheaply.

If you know what your minimum is then keep that in mind. They can either negotiate or say that is what they are paying. Then you can decide what to do.