Like others I think this was a throwaway line (which I admit as a former babysitter I found funny) but in fairness to the writers they've spent a lot of time painting Howard as a spendthrift. Even in the episode with the 3-D printer at the end he had to carry a packed lunch because, as he told Raj, he wasted his lunch money on comics 🙂. I think the message, constantly, is that Howard is a 12 yr old at least where money and household responsibilities are concerned. As I've noted though (and what a surprise to find a sitcom isn't consistent!) is that they don't always stick to this version of Howard - only when it suits the story.