there are too many variables to offer a price on such limited information provided by you so open yellow pages and pick 3 local small scaffold contractors to ask for a price.
have some information ready to assist them such as=
1) size is it a tower say 10" x 5" alongside the gutter to cut a hole in the roof and have a little bit of working room to stack some materials
2)length of time required
3)approx. height to platform
4)can a 7.5 tn truck gain reasonable access or is it a job for a pick up truck or not easy access and the materials for the scaffold have to be carried a long way by manpower
starting to get the picture? the other person does not know your home and needs some info to make an educated estimate.you will probably only get a firm quotation from a site visit, if they are prepared to spend their time doing so
good luck with the job