The honest answer is that height drives cost more than any other variable, and it does not scale linearly. Doubling the height of a piece roughly quadruples the material and can multiply the engineering by more than that, because wind load grows with area and with the lever arm at the same time.
The second driver is finish. A mirror polish on a three-metre stainless piece adds around three weeks of skilled labour, and labour is most of the difference between a satin and a polished quotation. Corten, by contrast, has no finishing cost at all — you are paying for the plate and the fabrication and nothing else.
Third is complexity of form. A sphere and a figure of the same height and weight are not the same job. The figure needs modelling, moulding, sectional casting and hand chasing; the sphere needs rolling and welding. Anything requiring an internal armature calculated as a structure — a horse on two hooves, a cantilevered wing — carries engineering cost that a self-supporting form does not.
What is included in our quotations: design development, structural engineering, fabrication or casting, finishing and export crating. What is excluded, always listed explicitly: freight, import duties, foundations and ground works, crane hire, and installation unless separately quoted. Foundations in particular surprise people — a six-tonne marble piece may need bored piles, and that is a groundworks contract, not a sculpture cost.
Payment is typically 40 per cent on order, 40 per cent on completion of fabrication against photographs, and 20 per cent before shipment. For anything above six figures we stage against milestones. If a supplier asks for full payment up front, ask why.