Wiktionary
poor as a church mouse
a. (context simile English) Very poor to a point of starving or begging. Expression based the earlier phrase, "hungry as a church mouse", derived from the fact that European church buildings in the 17th century and earlier did not store or provide food, and so a mouse in one was therefore utterly destitute.
Wikipedia
Poor as a Church Mouse
Poor as a Church Mouse'' (German:Arm wie eine Kirchenmaus'') is a 1931 German musical comedy film directed by Richard Oswald and starring Grete Mosheim, Anton Edthofer and Hans Thimig. It was based on the play A templom egere by Ladislas Fodor. The film's art direction was overseen by Franz Schroedter.
Usage examples of "poor as a church mouse".
But to do that he's got to have money and he's as poor as a church mouse, whatever a church mouse may be.