A million bucks or two would not buy a studio unless It's in It's early stages. Most studio buyouts would be large. Microsoft already did their big purchase years ago: Rare.
Obviously looking at it from the Rare stand point It's not worth the money to buy a studio, rather lure more companies into putting their games onto the 360.
I don't think money hatting is always needed either. I doubt Namco just put Tekken and Soul Calibur on to the 360 after Microsoft sent them millions of dollars, it was more of a case that the 360 install base was getting larger and staying exclusive on one system was not as profitable.
While It's nice to have a few highly touted studios under your wing to develop AAA titles I believe Microsoft doesn't see much profit in it, and are content with having few studios as a whole (Having the Halo name rights will generate them millions till we're all dead, Forza and PGR they own those names as well and can have other studios develop those titles if worst comes to worst. Let's see how well Bizaree Studios does with "Blur".)
It's just that simple.