It depends on both local law and the contest’s terms.

In general, prize money alone does not automatically mean you need a model release. Street photos may sometimes be entered or sold as art/editorial work without subject permission, depending on your jurisdiction.

Where releases are more likely to be required is when the contest can use your image for promotion, advertising, sublicensing, or in ways that could imply the subject endorses a product, service, or organizer. Many contests spell this out in their rules, and some also require you to indemnify them, which increases your risk if you don’t have releases.

Also be careful with editing or presentation that changes the apparent meaning of the image, since that can create additional problems.

Best practice: read the contest terms carefully and check the law where you shoot and where the contest operates. If the use is clearly commercial/promotional, get a model release to be safe. If it’s purely editorial or artistic, a release may not be required, but contest rules can still require one.