Fonti,
You may bought alu-body from hawkcars (and all other parts as well) or from kirkham europe. Then you may bought origin specs chassis (there is many people here who could sell that for you) or do it bye yourself and build you car from scratch. Or you could bought some used car or unfinished kit and fit alu-body to that.
See:
http://www.hawkcars.co.uk/hawk427/bodyshell.pdf
If this price info is still valid then alubody is about 12K€ or 17K$ (+VAT) so I don't belive that there is 30-40K$ difference between fiber/alu bodied car if the only change is body material.
Price difference between kits/cars comes from different reasons (suspension, chassis etc etc) as mentioned alredy here.