That's paying a company to develop the machine that can turn out your inflatable headphones in large quantities. And you need to warehouse them, and maybe even pay to have them shipped around the country to various distribution points. Oh yeah, I should mention you'll also need package design, logos, and someone to write and print the instruction sheets. You should also allow a bit of money for publicity. This might only be a few thousand dollars.
Publicity is different from advertising.
Advertising runs into the tens of thousands of dollars. For many, 'inventing fever' can have the same effect as playing a Las Vegas slot machine. More than one inventor told me that their gadget or gizmo becomes 'their baby', and once in a while 'the monkey on their back'. They wanted to see it succeed at any cost. Dozens took out mortgages on their homes, or borrowed money up to the limit on each of their credit cards.