Money Can Buy You Love -- But Only for a Bit
Howard learns this lesson when he falls for Penny's gold digger friend and uses all his bar mitzvah bonds to buy her presents. When the bonds run out, so does she ("The Dumpling Paradox," season one episode seven). In "The Wiggly Finger Catalyst" (season five, episode four), Rajesh Koothrappali is torn when his traditional Indian (and "Scrooge McDuck"-rich) parents insist he choose between his non-Indian girlfriend and his generous allowance. Raj, the eternal romantic, chooses love and tells her she has to return all his expensive gifts. She chooses to leave poor Raj.
In "The Rothman Disintegration" (season five, episode 17), Amy wants Penny to be her bestie so she spends thousands on a huge and creepy portrait of them. Penny hates it. Eventually, Amy finds out the truth but also learns she never needed to buy Penny's friendship.
Neither a Borrower Nor a Sheldon Be
Sheldon may be weird about banks and rarely cashes his paychecks, but he has thousands put by in a Green Lantern action figure. When Penny needs money to pay rent, he loans it to her. Although initially grateful, Penny starts resenting the loan and the lender as so often happens.
How did Penny get herself into this dilemma in the first place? She pays a huge fine for her ex-boyfriend. He promises he'll pay her back -- eventually. Meanwhile knight errant Leonard and his posse confront the hulk and, though humiliated, emerge triumphant, with Penny getting all her money back ("The Financial Permeability," season two, episode 14).
Don't Quit Your Day Job
All of them have safe university jobs at decent pay, save aspiring actress Penny with her one hemorrhoid commercial gig and day job at the Cheesecake Factory, but that doesn't keep them from trying a sideline or two for multiple streams of income.
However, the hard truth of entrepreneurship is pointed up when Penny invents Penny Blossoms, a hair accessory. The business is a lot harder than she expected. "You are effectively paying yourself $5.19 a day. ... There are children in a sneaker factory in Indonesia who outearn you, " Sheldon tells her after he breaks down her production costs. Sheldon, Leonard, Raj and Howard bail her out when they run an assembly line and get out her first big order ("The Work Song Nanocluster," season two, episode 18).
(Sales) Resistance Is Futile
The guys are suckers for their beloved comic and sci-fi collectibles or a Lego Midnight Madness sale. As Stuart, the store owner, says when they come in, "It's like shooting nerds in a barrel." Sheldon, despite his 187 IQ, is a sucker for anything Stuart deems "unique" or "limited edition," overpaying by hundreds for a squirt gun and various other "treasures" over the course of the series. Leonard, 173 IQ, convinces the guys to spend $800 on a time machine prop ("The Nerdvana Annihilation," season one, episode 14).
The girls don't understand wasting money like this. As Amy admonishes, "Sheldon, I'm disappointed. As a brilliant man, you're entitled to a vice. I could understand frequenting an opium den or hunting your fellow man for sport, but this is lame-o" ("The Flaming Spittoon Acquisition," season five, episode 10).
Talk Over Big Purchases With Your Partner
Sheldon can't decide between an Xbox One and a PlayStation 4 and so quite admirably researches it. But too much research has paralyzed his decision making. So he talks it out with Amy. Still, he overthinks it until the store closes, and he leaves without either gaming platform ("The Indecision Amalgamation," season seven, episode 19).
On the opposite end of the spectrum, Raj and Howard impulsively spend $5,000 to buy a used 3-D printer so they can make action figures of themselves (worth a couple bucks). Howard gives one he made of himself and Bernadette to her. But when she finds out how much the machine cost, she hits the roof because he never consulted with her. Possibly my favorite clip (from "The Cooper/Kripke Inversion," season six, episode 14), it also includes Raj telling Howard he deserves to buy the machine, "You worked hard to find a woman who makes a lot of money."
Buy in Bulk
The ladies have their own blind spots, especially Penny, with her collection of pricey shoes on a waitress' salary. On "The Spaghetti Catalyst" (season three, episode 20), Penny also teaches us to be careful with credit when she goes through her mail, "Damn, they cancelled my Visa. (opens another letter) Oooh...yay, a new MasterCard!"
Leonard tries to help Penny, but it's the practical Sheldon (in "The Luminous Fish Effect," season one, episode four) who advises Penny, "There a lot of advantages in buying in bulk." He's talking about 30 years' worth of feminine products. She slams the door in his face.