You might have seen this video of a gun buyback in Houston on Saturday: What’s in the box? A bunch of homemade pistols, helpfully printed in bright colors, probably still warm from the printer: Based on the buyback’s info page, those derringers in the box earned their maker either $50 or $150 each, depending on whether they were finished out enough to function.
This also creates a perverse incentive ala the "cobra problem." They're literally encouraging gun manufacturing. However, the only way to combat that would be to exempt "ghost guns" (a problematic strategy, considering their stated goals and ideology) or reduce the "rewards" to make 3D printed guns cost more than the reward, which is also a problem in that it reduces the incentive to turn in real guns.