I just did the same exact thing, 60 amp from the house to a 100 amp panel in the garage about 100 foot of wire box to box. I used #6 hots and neutral and a #8 ground (has to be green). When I asked my electrical supply house for white neutral wire they told me they just stock black and then...
You will need a separate ground bar and do not bond the neutral bar to the panel. Run the ground from the house to the separate ground bar and then run a ground wire of at least #6 to a 8' ground rod outside the building. You may need another 6 feet away from the first with a continuous wire...
That is cheap, if he is supplying the block and mud do it. I think I paid .95 a block or so for the 8's and 1.05 for the 6's on the top coarse. Mud was 3.50 a bag, I think it is 10 to 12 block per bag if there is little waste.
As long as you get your corners in correctly so the building ends up square it is not that hard. To lay 200 block you will want a couple people helping, one to help lay and one to tend the mixer it will make your life much easier with help. Good mud consistency makes a huge difference in laying...