The term cinder block refers to blocks literally made from wood cinders. Cinder blocks have not been manufactured for probably a hundred years. Blocks now are almost entirely concrete, hence the term cmu (concrete masonry unit). Generally the blocks used for retaining walls are solid. The blocks with holes are used for foundations, and are sometimes used for walls, but they are not used for segmental block retaining walls.
The important detail in the block is the presence of a key on each block. The male key fits into the female groove, causing each layer of block to offset back the correct amount, thereby making you build a slightly battered wall. So called cinder blocks (actually concrete block) do not have a key, and are not generally suitable for reinforced earth walls.
We built our wall with Ideal block, which is manufactured locally. We purchased a total of approximately 200 blocks, each block 6 inches high, 8 inches wide, and 12 inches deep, with a textured face that looks like stone. We also got some flat blocks for the steps, and some special blocks for the corners, but effectively it was $1000 for 200 blocks, or about $5 each. This is not much different than the quote for Versalok that you got.
Now to the question of what prevents the wall from sliding. This may be hard to wrap your arms around, but if you look at the mechanics of the wall, the soil behind the wall exerts virtually no lateral pressure on the block, assuming you build the wall according to manufacturer's directions. The soil supports itself. When the soil gets above a critical height, you need to install the reinforcing, but if you do it according to recommendations, the reinforcing supports the soil. Reinforced earth walls would hold themselves up without the block, however the block is there to prevent erosion of the soil, and to present an attractive appearance to the wall. Unreinforced earth walls (typically less than four feet tall) require some contribution from the block to hold themselves up. But in no case do you need to, or even want to, bury the block, install a concrete footing, or in any way deviate from the manufacturer's recommendation.
I installed my block directly on glacial till, no footing at all. No problem. The wall is only three feet tall, so it was pretty easy. I backfilled approximately one foot behind the wall with crushed stone (I used the same crushed stone for mixing concrete for my deck footers). No drain required in my case. Segmental block walls less than four feet high may not need a drain, see manufacturer's recommendations, because water will flow through the face of the block.
