If you can get a concrete pad 20x35 for under $2000, i envy your market and thats a definite go ahead. It is a one day job to prep the site and another to pour the pad, possibly a return trip to break down the forms also.
Like others have mentioned there are additives. I dont like additives because now youre turning the concrete into a witches brew and ive personally seen where that makes the concrete set up in the chute or take a week to set up. Thats the engineer in me.
Being in NJ, im sure you see the freeze/thaw like we do in CT. Have you considered an asphalt surface? Concrete driveways (and roads) are fairly rare around here because of how they hold up to the freeze/thaw, plowing, and salt we use. Thats just the engineer in me.
If you want the pad to last you are going to have to put a base underneath it of macadam (Never heard that word before, googled it and love it). Therefor, it will almost certainly be your cheapest route to do just a macadam surface. You could also try to find millings as a potential surface. A contractor with a machine and dump truck can have two laborers do that job in probably a half day if youre not far from the quarry. Faster and cheaper. If at some point you want to do something different, grass or concrete or Italian marble pavers, the material for the base is on site.