A Bird In The Hand Is Worth Two In The Bush: 
Having something that is certain is much better than taking a risk for more, because chances are you might lose everything.

A Blessing In Disguise:
Something good that isn't recognized at first.

A Chip On Your Shoulder:
Being upset for something that happened in the past.

A Dime A Dozen:
Anything that is common and easy to get.

A Doubting Thomas:
A skeptic who needs physical or personal evidence in order to believe something.

A Drop in the Bucket:
A very small part of something big or whole.