You could expect to pay about $14k for someone to supply and build a modular block wall of that size for you.

A proper poured concrete wall would have a wide footing extending several feet toward the uphill side of the retaining wall and it would be 4' or more deep (on the downhill side) and at least 8" thick. That would also make your poured wall 8' x 100' x 8". The concrete alone would cost $2,500. You'd need excavation, forming, drainage tile, steal reinforcement, and back filling. I think that you will find the modular block more affordable, especially if you do some of the work yourself.

It would be cheaper to cut a narrow trench pout a quick footing and put up 4' forms, but it will fail either through the freeze/thaw cycle or by failing to hold back the surcharge. The only question is which will happen first.

Thanks laag!
That is a lot of really good info! Since you are out on the Cape, I am guessing my costs will be similar (I am out in metro-west, near the Marlboro area).
I guess this wall will cost a lot more than what I thought. Sorry to keep asking question, but do you have any guess as to what the final cost of a proper concrete wall would be? Usually, I have found that a job will cost ~3x the cost of material; so I am wondering if $8k is a good guess, or if it will be even higher from all the labor (tying rebar etc)?

I'm out of touch on what it costs to set up and pour concrete forms, but it was about $15 per linear foot 18 years ago (not including concrete, excavation, and rebar).

Make a few calls and see what they have to say.