By contracts, do you mean licensing costs to third parties? I think day one launch releases are probably quite costly, but slightly older titles probably considerably less so.
Games ultimately have peak selling periods, after which heavy sales are often required to maintain even a semblance of the original launch sales, so older titles are going to be much cheaper just on that basis. Add to that, if Microsoft has data to show that Game Pass inclusion can add additional DLC and microtransactions revenue for third party pubs, that could offset any potential lost retail sales or value third party pubs think they might face.
That being said, I have no idea how likely it is people will buy DLC or microtransactions for older games, or even newer ones for that matter, and have to imagine the percentage of gamers that actually invest in microtransactions is fairly low, at least in the bulk of games anyway, but who really knows.