Tests cost money up front. Can you pay for those tests? You said you will pay for it in decreased costs once the business is running well.
Ok but how confident are you that your business will work out?
1%? you just increased your project costs 100 fold by doing tests.
50%? you just doubled your project costs by doing tests.
100%? Ok you are saving money, but if you are that confident then why haven't you gotten investors aboard that are convinced that doing it well from the start is a business edge?
That's not realistic.
Your last paragraph about overwork seems unrelated to TDD dogma.