Of course you don't pay taxes on Valve's share, but you can have 30% on your share removed, so something like 30% to Valve, 30% to the game creator, leaves you with 40% share, minus 30% taxes you are indeed left with 25-something percent of the initial value announced.
What's worse? If you aren't US citizen and have to cope with multiple tax system and currency conversions, you may very well end up with almost nothing while being in a low tax bracket.
I'll pass too, I never went into modding for the money anyway.