I don't know why the P239 should be less expensive. There is nothing obvious about its construction that differs. It may be a loss-leader to introduce people to SIGs, or it may have been designed to be more efficient to produce -- I don't know. I do know that I have never had a problem with one, nor heard of anyone shooting one apart, and that includes the .40s and .357s. I don't think durability or quality are an issue here.