Half day or full day? Regardless, it might be hard to find a Keys charter for less than $500. Some guys from work and I took a half-day charter in Hilton Head, and it was $400.
This service was recommended by one of the fly shops in town. (They do conventional tackle as well as fly.)
Rates & Bookings:Key West Angling
Also, a 15-20% tip is customary.