How much corundum is worth varies significantly based on its quality, potentially becoming anything from sandpaper to sapphires. Most corundum is opaque, muddy colored, and highly included. This material is industrial grade, getting used in things like abrasives, sandpaper, and power-tools. Industrial grade material is not even worth $1 a carat, especially with how cheaply other synthetic materials with similar hardness can be made.
Corundum that grows very similarly to its habit is valuable to specimen collectors, even if the colors are not particularly spectacular. Most of the time it looks very lumpy, and identifying the habit can be impossible in certain cases (like being a water-worn pebble). Mineral specimens that show ideal color and shape can go for millions at auctions depending on the quality.
There are different varieties of higher quality corundum. They are mainly grouped into sapphires and rubies, along with a shared set of grades. The lowest is highly-included to opaque carving-grade material, cabochan-grade material, and lastly facet-grade material. Amongst facet-grade material, only the clearest pieces are made into emerald cuts.
The maximum possible value for a piece of corundum would be found in facet-grade rubies. These rubies have exceeded over a million dollars a carat, and are always used in jewelry.