What is the ideal price for upgrading to double paned windows?

[Ashwin Sundar]

For my new investment, I'm trying to upgrade the windows. I have 10 windows to upgrade. I spoke to 2 different contractors.

Contractor A quoted $2400 for retrofit and $3000 for regular (labor only, no material).

Contractor B quoted $6000 for retrofit and $8500 (labor + material).

Are these good prices? Bad? How do I find good contractors? 

If anyone can recommend me any, that would be great too. I live in the Bay Area, California.