Well, the question I would first and foremost ask is: Why does he want to have those hearthstones?
Items like that always give me the feeling that they are created in anticipation of being screwed over by the GM, so you try to have an ace up your sleeve. If that is the case, I would try to remove this fear from the player, by simply giving it to him. He's got worldwalking magic, he's got the finest worldwalking magic in town, clicking his heels and getting back home is his easiest trick, he doesn't need anything special for it. It could even be part of his concessions. Instead of barely escaping with his life, he can escape back home, barely alive.
Because "getting home" will not be quite that advantageous in most situations. Sure, you're out of a dangerous situation, but now your enemy is free to act halfway across the globe. There's lots of more interesting options available.
If he wants to avoid problems during travel, that's quite easy as well: don't make any. Again, I have a flashback to old D&D times, where the GM would always throw in random encounters, just to screw with us. When it comes to traveling through the nevernever, 99% of the time, I use it as a convenient way to get my players from A to B in a short amount of time. And if I feel the need to attack them, I will offer them a compel, which they can either accept and gain a Fate point, or they pay one to remove the complication.
Ok, now that that's out of the way, I think I actually wrote something on the lightning portal some time ago. My idea basically went like this:
First, you turn the person stepping through into lightning. Transforming someone is something you can do as a taken out result, and since the target is willing, it won't take a consequence, so you can take them out with 5 shifts of stress (4 max stress track +1 to take them out. Since it's voluntary, that can count as the catch). Next, you do an athletics replacement spell for them, that basically replaces what they would have to run on their own. Since it's the nevernever, we can probably take some shortcuts, especially since the lightning can travel ways that humans wouldn't be able to, and at greater speeds. Still, I think around 10 shifts might be in order, but YMMV.
Turning people back at the exit doesn't necessarily have to be a taken out result again, the transformation can just be temporary.
And last but not least, the whole thing is set up as a ward, since it is activated when someone steps into it.
So a 15 shift ward could do the trick for the lightning door, but it's essentially the same for a hearthstone. Remember that magic still follows physics to a degree. If you move someone really fast, they are going to get whiplash, not to mention they'll smash into things, so you are going to have to transform them into something transportable first.
I think it's easiest to cut down on the 10 shift transportation fee for the wizard by taking the "Demesne" power. It would allow him to shape the nevernever to a degree, which in his case would mean that he could create his own bubble of the nevernever around the original stone and the hearthstones would be connected to the same part of the nevernever, only a step away. That would mean instead of 10 shifts, I'd reduce that to a mere 1 shift, so you are looking at a 6 shift item.
