Be onto a good thing
Definition 1 of 1
Pro Tip 1/3Positive Connotation
This phrase suggests a positive outcome, often with an element of luck or unexpected success. 
I can't believe the demand for my cakes; I'm onto a good thing here.
Pro Tip 2/3Informal Use
It's an informal expression more suited for casual conversations than formal writing. 
When she said I'm onto a good thing house-sitting for her, it felt casual and friendly.
Pro Tip 3/3Not Always Financial
While often about financial benefit, it can also refer to other advantageous situations. 
Studying with her has improved my grades; I'm definitely onto a good thing.