I think it's a bargain for a few reasons. Horizon is an older ship and they tendtobe priced more aggressively than the newer, larger ships. Also Tampa has only a handful of ships leaving from there and Celebrity was a recent addition.
I sailed Horizon a long time ago and thought she was really lovely. I believe she is doing a Western itinerary which I prefer to Eastern. Also Tampa airport is a whole lot nicer than Miami and a quick ride to the port.