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I don't think it's necessarily a cost savings... NPT or BSPT is used pretty much the world around and you don't take a cost hit versus BSPP. But the tapered thread (NPT or BSPT) are interference threads and in plastic have a somewhat limited life before they start leaking. The seal is made by jamming the thread material into each other. Using a parallel thread with a gasket (BSPP) gives you a joint that's secure and leakproof that doesn't depend on material giving way - the seal is made by compressing a replaceable gasket. Granted, BSPP is a difficult fitting to find in the US, but from an engineering standpoint is probably the better choice.
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