With an eye for/on/to the main chance
Definition 1 of 1
Pro Tip 1/3Business Acumen
Refers to someone's ability to see and seize business opportunities. 
She has an eye for the main chance, turning a small startup into a profitable company.
Pro Tip 2/3Opportunistic Nuance
Conveys a sense of being alert and ready to take advantage of situations. 
With an eye for the main chance, he bought shares before the company announced its success.
Pro Tip 3/3Potentially Negative
Can imply someone is interested only in their own benefit, possibly being manipulative. 
He's with an eye on the main chance, often neglecting his colleagues' ideas.