Same fuel tank cap is used on GL1800 and NSA700A (DN-01): 17620MEH003 (replaces older part numbers: 17620MCAA01; 17620MCA003; 17620MCA013; 17620MCA780).
Price in this Netherland online shop seems a bit less than they said to you: https://www.cmsnl.com/products/cap-assy-fuel-tan_17620meh003/
Same fuel tank cap is used on GL1800 and NSA700A (DN-01): 17620MEH003 (replaces older part numbers: 17620MCAA01; 17620MCA003; 17620MCA013; 17620MCA780).
Price in this Netherland online shop seems a bit less than they said to you: https://www.cmsnl.com/products/cap-assy-fuel-tan_17620meh003/