There is always a risk with exclusives. I agree though. The big console makers have a bigger wallet to take risks with exclusives in order to prop up their platform. Valve is a private company though, their finances are not really public. They could be sitting on billions for all we know. I doubt it though, since their Steam commissions aren't very high.
Who really knows, but it keeps growing every year. If that $55 million in operating profit in 2005 is accurate, they're doing damn well by now. It's not as if MS is raking in profits from the 360 until recently, and considering the expenditure and the fact the original Xbox never made any money, Valve may actually be more successful - albeit they don't have a massive organization like MS to back them up of course.