It's a few acres of prairie with some buildings that need to be torn down. I'm not up to date on land prices in western SD, but I can't imagine good pasture land is worth more than $2k/acre, even with the surge in beef prices recently.
It's also worth noting that the Pine Ridge and Rosebud Indian Reservations are located in the poorest counties in America. I've visited a couple times and it really is striking that such poverty exists in America. The one stat I can remember from a volunteer trip several years ago was that an 18 yr old boy had a <50% chance of living to 25. When I was in high school we would play football and basketball games there on occasion - our bus always had police escorts in and out of town.