I would have hoped that when Delta was able to fly RJ's into Key West (vs TurboProps before, and still on other carriers), they would have been able to find a way to do it to lower the cost of flying there. Haven't seen it, prices seem as high as every. (PHL-EYW is typically $400+, whereas PHL-FLL I was able to get on DL 10 days ago for $128 R/T).
I guess the fact that there are only 5 RT flights to/from EYW (2 from ATL, 3 from MCO), plus Key West being a sort of expensive place to visit, doesn't otherwise lend itself to lower fares.
Jeff