[E-40six]

To be honest, I believe 6000 for the job is a decent price

People fail to realize that the disposal part alone would be easily $1000.
because you're going to need 2 separate roll off bins for your job. You'll need a 10 yard concrete bin, and a 30 yard demo bin. Those bins alone will be easily $800-$900 if you do not go overweight.
If you dont want to rent a bin, you can load up a pickup truck and make multiple runs to the transfer station. The true disposal fee for all of that is in the $300-400.00 range. you'll also need to factor in the truck rental cost and your time.

Once you finish demolishing the structure and concrete pad which would most likely need a skidsteer with a jackhammer attachment. You could get it cheaper if your contractor is just going to sledge it with a couple of guys. The trees will have to be removed including its roots system. This has got to be the best part, pulling roots!

Once you get all the fun smashy stuff out of the way, you will need have the area dug down to accommodate the road base/crush rock and then have it graded so water properly drains. Roadbase is cheap, you can get buy for approximately $5/tonne but the trucking fee will be around $100-150. Crush rock is expensive at $35/yard and you will also have to pay delivery on that.

after all that you can start your pour and once your concrete is dried, you will need to seal it. With all materials considered, the contractor is hovering around $2500ish in expenses before labour and equipment rental costs.

and remember, contractors aren't out to make an hourly wage they are a business and you need their expertise to have everything done properly otherwise you'll have a cracked pad after a year or 2.