Not bad, but both Jamie and Adam were well respected visual effects artists, builders, and model makers before Mythbusters. Mythbusters primarily shot at Jamie's shop.
Grant and Tory both worked at ILM. Kari was an employee of M5, Jamie's company, when she was put on the show.
So, not terribly rich, but about what you would expect. $1 - $2 million "net worth" is more attainable than you might think. As net worth includes assets and what not. They have enough money to take substantial risks that might not pay off.
Given the net worth of Grant, Tori and Kari, I'm guessing they didn't have huge salaries from the show, maybe $200k/yr on average.
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.
Jamie and Adam were stars of a wildly popular television show that ran for 14 seasons, spawn numerous spin offs and runs in syndication even today. I'd expect them to have hit at least $500k per episode.