Think about what you spend money on, and the people behind it. They might be making your lunch, bagging your groceries, or mowing your lawn. They are probably mediocre at it, but you pay them anyway.
Others will pay you decent money to do a mediocre job, and good money to do a good job, because you are the exception to the rule of being willing and able to do the thing for them at all. Most people who have problem X would rather pay money for you to solve some problem for them (even if your solution is mediocre) than solve it themselves, because that allows them to focus on solving whatever problem they care about or think they can get paid for.
Also, look at corporations in general. A lot of them are inefficient and it seems like a miracle that they can make any money at all. Yet they clear big bucks. If big lumbering TPS-churning corporations can create value, then so can agile me. If someone whose job it is to shuffle papers and write memos can make a large salary, then why shouldn't my command of programming be valuable?
All in all, I think it's better to have the belief "I am worth lots of money" than the opposite.
Others will pay you decent money to do a mediocre job, and good money to do a good job, because you are the exception to the rule of being willing and able to do the thing for them at all. Most people who have problem X would rather pay money for you to solve some problem for them (even if your solution is mediocre) than solve it themselves, because that allows them to focus on solving whatever problem they care about or think they can get paid for.