I'm sure the cheap fares for the dates you wanted either were a sale that ended or they sold out. Airlines price to maximize their revenue so if a flight is popular, fares go up. This is another good example for all the people who ask "should I buy now or wait" that fares are as likely to go up as down and when you see a fare you like, buy immediately.
Also, check on kayak.com. For domestic USA flights, they will give you a prediction if flights are going to go up or down by comparing the current fare to history. You'll be able to see if the $316 fare was abnormally low or if the $811 fare is abnormally high.
Also check alternate airports. Can you drive to Chicago or Des Moines? Can you fly into somewhere else in Florida?