Bought TPMS sensors for new wheels from Discount Tire - the clonable type. Work great. $45 each.
Still way more than a passive system ($0). They obviously could have gone with a passive system as well, but that wouldn't report tire pressures on the roll. I think that feature is what they were after. Given the amount of attention paid to tire pressures here, I suppose there might be a following. I personally think it's not worth it.
I don't need to car to beep at me when my pressures reach the level it thinks is good. That's what tire gauges are for. And yes, I check pressures on all my cars regularly, not just on the Leaf. Prolly makes me a unicorn I guess.