*Of course* OSS was used as a tool to lower prices MS prices.
Face it: if you were about to spend lots of money, and the vendor in question had a public argument with others about the price and use of its product, why would you not try this out ?
It's common knowledge amongst corporate buyers that the software vendor's sales people use any and all means to grab your money. After all, all the salesperson has is talk : the product he sells doesn't cost his employer more than a few $.
Microsoft can send a single box to a 50,000 person company as well as to a 1,000 person company - the price for the vendor is "per company" not "per seat". But they'll pay a different price for that box. Ask yourself why. Remember that on top of the license fee, they'll pay a lot of $$ for support (which does depend on the amount of seats).
In the end, the licensing you pay depends on how good a negociator you are, and how good a salesperson the vendor has.
So once again - if you're buying and you want to do a good job, you'll use all the tools you can get your hands on. The OSS community is just one of them.