That is all they need to do is write a check. It would be a steep check. Gabe could start another company and do it better than steam. And if Microsoft wanted to Xbox live the steam service then the next stream would become the go to place for nerds.
Gabe already did that and has everything in place. Microsoft looked at PC gaming sideways and he left to make Valve.
Valve is still privately owned and in his control. His dislike of Microsoft prevents a sale, and any money they throw at him will just end up being gained in a number of year's time without a sale.