You don't even know what you don't know.
Pot, meet kettle. I'll use "PC" as shorthand for "PC/Mac/Linux" in the following. Here're some of the sales figures I came across in a quick search.
Bastion: ~300,000 PC sales in it's first year. Total sales were 5/3rds of this, which we'll use to calibrate other XBLA/PC titles. ~1.1 million PC sales over two years (3/5ths of 1.7 million total)
Super Meat Boy: ~1,200,000 PC sales over three years (3/5ths of 2 million total)
Amnesia: ~1,400,000 PC sales over three years.
Grimrock: ~600,000 PC sales over six months.
Binding of Isaac: ~1,000,000 PC sales over a year
Hotline Miami: 130,000 PC sales in two months
Yeah, these aren't $40 games, but y'know what? If PE/W2/SR/etc are critical darlings but hit this million-strong audience only when reduced to $20, $15, $10, they'd
still be roaring successes. A million sales at the $15 price point for PE would still yield ~$10 million profit for Obsidian.
You seriously misjudge the size of the PC market. Personally I'm expecting ~500k at $30 and another ~million at $10.