Novelty play money and professional prop notes can look similar at arm's length, but a camera is less forgiving. Novelty notes are often smaller, single-sided, or printed on glossy paper that throws glare under lights. Professional prop notes are printed in high definition on both sides on matte archival paper, so they read well in a close-up and handle naturally in a counting scene.
There is also a compliance difference. Every PROPPS note carries clear specimen markings and is made to stay clearly non-legal tender in line with the Crimes (Currency) Act 1981, which keeps a production on the right side of the law. Novelty money sold without markings gives you none of that.
For any scene where the money is a central plot point, spend on professional notes for the hero shots and use simpler stacks for the background. See the New Notes range or the Event & Party range if the money is for a game or a party rather than a shoot.