Valve had the advantage of Newell/Harrington starting the company with a pile of money from being former MS staff, which gave them the ability to say "when it's done" for their first game. As the resulting first game was a hit and they didn't owe much (if anything) to their publisher Sierra [1], from that point on they've kept rolling the success forwards. Being a private company there's no details I'm aware of on how much has been taken out of the business, although Harrington left not long after Half-life.

[1] I vaguely remember a legal dispute when Valve did release steam years later as Sierra were being cut out.