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.