What affects the cost of a concrete driveway installation?
As you've seen, prices vary widely.  That's why it's a good idea to shop around and ask for references.  Some operations just don't want to bother with small jobs like yours, so they quote you a high number in the hope that you'll pick someone else.  If you pick them, you pay for the &quot;privilege&quot;.  Some quote low-ball prices and I'd be wary of them.  Yes, there are dishonest outfits.  They may use shoddy materials and do crappy work or they may have &quot;unforseen&quot; issues that will cost you more once the job is started.  You name the scam and there is a contractor who does it.
If if were me, I would look closely at those that are in the middle which is where things should cost.
Also, in most areas there are only a few concrete suppliers and their prices are usually quite close.  Call and ask their price per yard delivered for 4000 psi mix.  From that and the number of yards needed you should be able to estimate the concrete cost.