A big notable example is Square Enix's Final Fantasy 13 trilogy (3 games, FF13, FF13-2, and Lightning Returns), where IIRC sales basically halved on each release - so Lightning Returns sold about 0.25x what FF13 did [1]. (FF13 sold well, so this is at least not completely dire).
You can often make this up by spending less money to develop later episodes & sequels, which was true for both Telltale and the FF13 trilogy - but eventually it's not sustainable anymore.
1: http://www.siliconera.com/2013/11/27/lightning-returns-lowes...
Which makes sense, now that I'm hearing how the company was managed - coming up with really original ideas, and making sure they work, takes time and breathing room. You get that out of people that have time to put their feet on the desk and daydream, not people that live on perpetual crunch time.