I'm not sure where the cutoff is for a aftermarket CAI manufacturer to decide what is, or is not, worth building a design for sales. I do know that Ford very specifically bragged that 1 in 5 Explorers sold was an ST. If you go by last years sales figures of 180,000, that is definitely a huge seller for what should be a "niche" sport trim.
To the cost vs. benefit take, i do agree the costs on intercoolers and downpipes are high, but, with a tune, those can provide significant gains, with the stock air box. So i think people are willing to opt into those and not care about a CAI. And maybe that's what the CAI manufacturers are looking at and deciding it's just not worth it.