2nd Charge Moves Costing Less on Lower Evolutions

In the "Tips & Tricks" section of the "Pokemon Go Trainer Battle Guide" it says 2nd charge moves cost less on lower stage evolution pokemon and carry over, do we know for which species? I checked a couple costs - Dratini & Dragonite, Swinub & Piloswine, Rhyhorn & Rhydon, Bulbasaur & Venusaur, and they all seem to have the same costs in their respective evolution chains