How many bags of mortar do I need per number of 8x8x16 concrete blocks?

I want to build a small basement, know the number of blocks I will need
(around 800), but dont know how to determine the amount of mortar. I
intend to buy raw mortar and add my own sand. I'll just buy a truckload
of sand, which I assume should be more than enough, but how do I
determine the number of bags of mortar? This is a rural area and I have
to drive 55 miles to get it at a reasonable price, so I want to be sure
I have enough. I refuse to buy it locally and pay their outrageous
ripoff prices (for example, the blocks at the building center 55 miles
away are $.89 each, locally, they are $2.40 each).

Thanks