Seems like the cost has come waaaay down since I looked into buying a bunch of these. IIRC, from HESCO, the big containers were around $500-$600 for a unit.
One thing to notice is that the alibaba link lists costs as xxx.xx FOB. The FOB generally means that the shipper picks up the cost of transporting it to a port and putting it on the ship, but after that, it's on you. You cover the cost of the ocean transport, customs, offloading, and delivery to where you want it. I'd guess minimum of $2k in shipping and freight costs, probably closer to 4k for a 40ft conex.
Also, if you build out of the big boxes, the rain will help tamp down and settle your fill. Once it has settled (a few rainy days), you can top-fill with concrete to level it off smooth. When we stood up a FOB in downtown Kandahar, we had the contract laborers lay out and fill the HESCOs, then soaked them with a couple of tanker trucks, and then top-filled with cement. Then we put a second layer of smaller ones on top, filled those with dirt, and stuck poles in them to hold concertina wire. Then we zip-stripped concertina wire to the outside of the HESCOs three stacks high around the outside. Basically, we had a wall that nobody wanted to climb, topped with a row of 3' high HESCOs that we could use as cover to shoot from the top of the wall.