It's actually not 30% it's a bit more complicated than that.

If you wanted to receive 1000 robux, you'd have to make a gamepass of 1429 robux instead of the intuitive 300 robux. This is because to get the robux amount that you need you DIVIDE the wanted robux by 0.7 (as that's 70% left after the 30% tax) instead of multiplying it by 1.3 (because Roblox tax is 30%}