How much dust and how many candies would you spend for a 2nd charge move?
With legendaries, it's going to still be a game of candy for me.
My guess is less candy and more dust. Perhaps 10-25 candy and 10k-30k dust. For a legendary where candy is very difficult to get, having to spend 50 candy for a second attack seems really hard. On the other hand, for something like pidgey and it's evolutions, 100 candy would be a tiny cost. Hopefully they get this right. Want to have a Bite/Crunch/Stone Edge Ttar that can do double duty, and Rayquazas with a double fast move could be very fun.
It depends a lot on the pokemon, but I would guess that the absolute upper limit it could be is somewhat related by how much it costs to power up another copy of the same pokemon.
For most pokemon I would spend 500 candy without blinking to unlock a second charge move and for the rare species I'd consider 50 to 100 candy to be the upper limit above which it becomes truly painful.
For dust, I guess I would shell out 100k. If it was 200k per pokemon I would maybe do it with a few favorites/best pvp meta species.
But I don't expect either candy or dust costs to be high enough to cause me trouble, as these kinds of numbers would be completely unfeasible for more casual/rural players who aren't rolling in resources.
A quick thought around this made me realize I don't care much what the candy cost is unless it's over 200. For me it's much more valuable to have 1x Pokémon with 2 charge moves than 2x Pokémon each with a different charge move like up until now. It is more aligned with the love and care you put into training your mons in the main series (far from maxing out teams of 6 identical ones, like the raid dynamics require for most challenges).
Plus there's the stardust cost. I would pay anything that is lower than bringing up a standard L20 raid catch with lucky feature, so around 80k is still alright for me.
And yet, my gut tells me it will be a lot cheaper, like the comments above. So for me it is probably going to be only good news.