"DIY Dudes" made a DIY crossbow with aluminum rails and PVC pipe bow but the trigger mechanism is fully 3d printed. The trigger group is the most complex part of the crossbow and as video shows it is fully operational.
There is the video of the crossbow in action:
The files for the trigger mechanism were not released but you could reverse engineer them from the drawings.
If you are interested in making a PVC crossbow for under 10 USD here is a step-by-step guide video by Nicholas Tomihama: