Excellent question, the answer could fill many many pages but I would say it depends on the robot and what you need it to do.
We have built robots out of straws, balloons and a couple cheap bits of electronics so maybe £10, other robots that have bought were closer to £250,000.
We normally have to count our time, materials and equipment to really figure out the cost but just costing for the parts it the easiest. The current robots we use cost £25 for parts and the robot we built to carry them cost another £500.
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