So can you tell me what the long-term effects of wearing this $45 printed device are?
Is it weighted such that it pulls muscles awkwardly, causing pain after a few months of continuous use? Does the constant contact with skin cause any nerve damage? If worn during physical activity, does it create an additional risk of shattering or otherwise injuring the wearer or others?
Can you show test results indicating otherwise, even when the user may not have it attached properly? What resources are available so the user can be certain they're properly fitting the device?
Approved medical devices are expensive because they meet all applicable regulations, and have documentation to prove it. They've been reviewed and tested by experts in the field, who understand exactly what subtle problems to look for that are likely to cause harmful effects in the future. One of the primary principles of medicine is to do no harm. Can you assure patients that this 3D-printed model will be harmless?
Yes, you can buy a beat-up used car for $500. It will still accomplish the obvious goal of transporting you from point A to point B, but it's not going to be as good in the long run as a more expensive one.