The devs already said that every time new talents will be introduced (or old talents modified) they will give some free respecs.

I really don't see why respec cost should change: just plan ahead and you'll be fine. I used one respec just to get the achievement, and basically bought the same talents I had before... 3 free respecs are more than enough, just plan ahead and optimize you talent choice. If you can't get it right by the fourth time, it's more than reasonable that you should have negative consequences, like an idol loss. I wouldn't even give the chance of respecs beyond the free ones...

(PS: it's respec, as in "re-specification", not respecT)

First of all, you shouldn't need to respec talents at all. I explained why above, if you can't do a little optimization a game like this is not for you. If I was one of the devs, you would get the free respecs and that's it.

Why a percentage cost instead of a fixed one, either idols or rubies? Because a fixed cost has a much higher impact on new players. 200 rubies are nothing for a player that has already opened 300 jeweled chests, but are a lot for someone who has opened 3. The same goes for idols: I can lose 1000 idols and hardly notice, while there are players who don't even HAVE 1000 idols. The cost should not be the same for all players, percentage-based is the right choice. Well, actually the right choice would be to allow no respec at all after the free ones...