With the 5K, you're paying for the Vera Bradley bag. Big Time.
As for the 1/2, it's in line with the entry fees for New Orleans Mardi Gras, Apalachicola, and Birmingham, but significantly more than what you'll pay to get to the line for Pensacola, Tallahassee, Warner Robins, and if you're looking for a cheap half Timberlake at Eglin (we promise paved roads for this one) right around Thanksgiving will set you back only $20 (though I don't know if that gets you a t-shirt)
As for most expensive, they aren't close. Disney will charge you $135 for their half, though that comes with a hug from Mickey at the finish line.