Hi there! Someone commented that this thread was here over in my little youtube video there, and I saw the question, so I decided to reply.
The trombone was made of around 1-2 lbs of green ABS plastic, which I purchased at $70 for a 5lb roll. So call it $28-ish for plastic, which includes the rejected prototype pieces along the way, and the material removed from the raft segments I used for printing the long skinny tubes. Actual cost of the final printed parts was probably closer to $20, then there was another $0.50 for the heat set insert and cap screw for the water release valve at the end of the slide, and probably $7 worth of PVC, PEX and glue. Also 12 hours worth of electricity.

I took the finished trombone over to play it next to a real one, and it does sound a bit more muffled than an all brass unit. It's also harder for some reason to reach the mid tones on the plastic one than it is on the standard brass based unit. It is plenty loud though. You lose the resonance that the brass provided, but airflow itself seems to be fine. The slide also has a tiny bit more friction than the metal equivalent, but still certainly in the playable range. The 3d printed mouthpiece definitely needs some work, as the shank on it is way too fragile and the inner cup isn't quite the ideal shape.

I'm actually working on a 3d printed trumpet as well. The valves are the sticking point on that one (pun intended) but asuming I can get my hands on a real trumpet to see how the pathing should look, that may be a reality here in the next few months too.

Glad you enjoyed the Trombone!