Sugar content is about 18% with proper applications of fertilizer and herbicides, as 9% SHTF would be optimistic. There is a huge investment in setup labor, mainly thinning, hand weeding, and harvest. Next is the labor and resources needed for extracting and boiling down down the syrup. You are going to need to utilize the remaining 90% of beet scraps for at least animal feed. Once you are producing more than adequate foods crops, sugar beets might be considered a luxury or trade crop.
I have known a few people who have grown sugar beets and attempted to extract the sugar. None thought it was worth the effort, that is now not under SHTF conditions. There was a reason sugar was a luxury for such a long time.