wouter,
This price would depend on the final design, ie are the hulls sheathed in glass? do you use stringers in the chines or fillet and tape? etc, I would expect you to be able to build a pair of hulls for around $500 Aus.

You're paying around $40 to $60 per sheet of 4mm ply and there is 3sq meteres per sheet. You will probably need up to 4 sheets. $6 to $10 pre Metre sq for glass. $15 to $20 per litre of resin depending on the brand and size it is bought in.

I think you will find it cheaper and lighter but more time consuming to fillet and tape the chines.

Once you have a final design you should be able to get a better idea.