Then you go after the publishers next.

It's clear that Valve's cut has an influence because it being a % it means that as development costs go up the price of the games need to increase exponentially to compensate for the 30% Valve gets no matter the price.

To quote myself for some numbers:

It you need to make 30$/copy to cover costs in 2015 and Steam is taking 30% you need to sell for 43$/copy, Valve is making 13$/copy.

Development costs go up by 20% over the next 10 years, you now need to make 36$/copy to cover costs, with Steam's cut you now need to sell for 51.50$, Valve is making 15.50$/copy.

If it was 15% instead? 35.50$ and 42.50$ would be the prices.