The costs aren't 30%.
Assume a $50 game. 100,000 sales. 5 gigs.
Storage and bandwidth = $28,000 (per Amazon's calculator)
Cost of doing business in N countries = use paypal = 2.9%+0.30/sale = approximately $1.75/sale = $175,000
Costs = $203,000
Revenue = $5,000,000
Ratio = 4.1%
So yeah, Valve is not the shining knight here and that 30% fee is unjustifiable. Even Epic's 12% share is still very much profitable for Epic.
The 30% fee is also nowhere comparable to wholesale pricing schema of the retail model (effectively a 40-60% fee), since that involved the sale of physical copies of goods, and the retailers had to deal with issues like shrinkage and the risk of unsold inventory.