Quick Answer: As one would assume, the characters on The Big Bang Theory all do pretty well. Their respective salaries are never stated outright on the series, but based on various data found on the internet of median salaries within the characters' given professions, we can infer most of them are making close to or over the $100,000 per year mark.
By now, everyone knows the principal actors on The Big Bang Theory (2007 - ) are raking in $1 million per episode for their performances as Sheldon Cooper (Jim Parsons), Leonard Hofstadter (Johnny Galecki) and Penny (Kaley Cuoco). With one episode airing per week, that’s a hefty chunk of nickels. For contemplation, a million dollars broken down into a standard 40-hour work week is $25,000 per hour. That's a little bit higher than their in-show characters are likely bringing home from their jobs at universities, science labs, and pharmaceutial companies.
The characters they play might not be mega-millionaires, but Penny aside, the folks of TBBT have never appeared to have too much trouble with money. Even the fake people played by Parsons and company look like they’re better off than the typical American, with fancy computers, spacious Pasadena apartments packed full of trinkets and collectibles, the ability to travel wherever and do whatever they want, and even purchase the occasional replica time machine. It raises the question: as highly-regarded scientists and professionals working for big universities and research companies, exactly how much would these characters be making?
Jim Parsons as Sheldon
Dr. Sheldon Lee Cooper, B.S., M.S., M.A., Ph.D., Sc.D., is an experimental physicist at the California Institute of Technology. During the first few seasons of the series, he was being paid by a grant of unspecified size to research string theory as a senior researcher. When the grant money ran out, the university made him a junior professor so they could continue to pay him, changing his focus to dark matter and stipulating he teach a class on analytical mechanics. The Chronicle, which has academic salary records for schools across the nation, puts the 2014 salary of an Associate Professor at CalTech at about $142,000 per year. That explains why he had a few thousand dollars cash in his desk drawer ready to hand his assistant so she could buy Amy (Mayim Bialik) a birthday present.
Johnny Galecki as Leonard
Dr. Leonard Leakey Hofstadter, Ph.D., is also an experimental physicist at CalTech. Leonard doesn’t teach class and his renown in the world of science is somewhat lower than that of Sheldon, lending to the assumption that he doesn’t have quite the salary of his best friend. Glassdoor and Indeed suggest the average staff scientist salary at CalTech is $74,920. Research scientists there average $89,981. Salaries for research scientists in Pasadena as a whole are in the $50,000 to $115,000 range, and CalTech would tend towards the top of that scale. It is probably safe to assume Leonard sits in the $90-115k range.
Kunal Nayyar as Rajesh
Dr. Rajesh Ramayan Koothrappali, Ph.D. (Kunal Nayyar), is an astrophysicist in CalTech’s physics department. He discovered a planetary object beyond the Kuiper belt, 2008 NQ17 (which he calls "Planet Bollywood"), for which he was included in People magazine's "30 Under 30 to Watch," granting him some form of celebrity status and a larger audience for his work. In 2010, the U.S. Bureau of Labor Statistics put the median annual salary of American astronomers at $87,260. For those working at universities, the median annual salary for astronomers working in research and development was $92,040, and Rajesh works at a renowned university. One can safely assume his salary is teetering around or above the $100k range.
Simon Helberg as Howard
Howard Joel Wolowitz, M.Eng. (Simon Helberg), is an aerospace engineer and astronaut. Aerospace engineers design aircraft, spacecraft, satellites, and missiles, and Howard has also been tasked with responsibilities like piloting the Mars Rover and physically traveling to the International Space Station. The average salary for his specialty is about $103,000, but he likely brought in even more during his travels to space. He would realistically have one of the higher salaries of all the cast members, despite the fact his wife Bernadette regularly reminds him that she makes significantly more than he does.
Melissa Rauch as Bernadette
Dr. Bernadette Maryann Rostenkowski-Wolowitz, Ph.D. (Melissa Rauch), started the series as a Cheesecake Factory waitress with Penny. She earned a Ph.D. in microbiology and achieved a high-level job at a pharmaceutical company named Zangen. Her salary has been referred to on the show as a “butt load” of money. It’s hard to say how much she makes, as a private sector pharmaceutical job could provide a huge range of potential income. Needless to say, between her and Howard, the Wolowitz family is doing alright.
Kaley Cuoco as Penny
Penny Hofstadter spent most of the series as a Cheesecake Factory waitress making minimal restaurant wage plus tips. It is frequently noted that she is a bad waitress, furthering her poverty as someone who likely received menial tips from her patrons. She also spent many years as an aspiring actress, appearing in only a few low-budget B films that produced little income. Her acting credits include a topless scene in a film called Serial Ape-ist and a production of The Diary of Anne Frank, which took place above a bowling alley. Now she is the third best salesperson working as a pharmaceutical rep for Zangen, the same company that employs Bernadette in a research capacity. Glassdoor suggests the average salary for a pharmaceutical rep in the Los Angeles area is just shy of $74,000. If Penny is in the top three of the company and $74k is the median, one can assume she is pushing above that number.
Mayim Bialik as Amy
Dr. Amy Farrah Fowler, Ph.D. (Mayim Bialik), is a Harvard-educated neurobiologist. Where she works has never explicitly been stated; she has a laboratory and is regularly seen conducting experiments, but the location is unknown. It is also unclear whether it is a corporate setting or a university. If it is a university, it is not CalTech, as the CalTech guys have traveled from one location to the other on previous occasions. The Chronicle puts a full-time researcher at UCLA’s salary in the high $60k range, but given we don’t know the capacity of Amy’s employment or the true location, it’s a hard guess to make. We do know she spent $3,000 to have an oil painting of her and Penny commissioned for Penny's birthday.
It appears one constancy among all the characters of The Big Bang Theory in their existing occupations is an above-average income. In 2015, the median household income in the United States was $54,462, or substantially less than every TBBT character makes as an individual. In a way, the characters on the series mirror the real-life actors that play them as people making more money than everyone around them.