Is PVC tubing a good material for helicopter airframes?
No. The strength to weight ratio of PVC is poor compared to aluminum and you would end up with a very heavy, albeit cheap, machine, and will be even worse when compared to carbon fibre tubing, which would be the optimal choice performance-wise.  Extruded aluminum tubing would likely be the best choice cost wise.  Forget about building a plumb-O-copter.