Is it actually? Do you have particular sitcom(s) in mind? I tried to look for this myself and couldn't find much (beyond ratings for anything beyond the top ~20 highest rated shows in a given year).
I know very little about this, but I'm a little skeptical because I feel people tend to vastly overestimate how much money the average person they see on TV makes. The stories of the outliers (like the Friends stars making $1M per episode) really skew the perceptions of this -- the vast majority of professional actors (if not regular TV actors, but maybe even then) make less than the median Bay Area software engineer. A salary on the order of $200K a year for a non-headliner regular actor in a sitcom with viewership around 1M people (which I think is more than a typical episode of Mythbusters would've gotten) doesn't sound that far out of the ballpark to me.