Compared to the cost of the entire project. The expensive part of the baskets was time honestly. They come precut and halfway put together, but building them out into a box shape, supporting the sides of each one so they don't budge out, and then lacing them all together was almost 25% of the labor on the project.
For approximately 200 ft of wall averaging a height of about 9 ft, the galvanized steel baskets alone cost me approximately $10K.
6 bucks a sqft for a retaining wall doesn't sound to terribly bad. I've seen them go 8n before. Definitely a lot of labor