Found this old page .... "digital film cassette" for regular 35mm cameras.... only US$ 1000 per cassette - 1998 price ....
IMAGEK EFS-1 - Every 35mm Camera a Digital Camera
EFS-1 looks like a regular film cassette, with a rigid tongue of 'film' sticking out, which fits into the camera's film cavity. Mounted on the rigid tongue is a 1.3 megapixel (1280 x 1024) CMOS sensor which sits in the film plane. Inside the cassette is 40MB of storage and a standard camera battery.

IMAGEK EFS-1 - Every 35mm Camera a Digital Camera