Perhaps the most accurate means of estimating cost in your area is to get in touch with the people suggested, Mssrs Eiley, Alfaro, Graniel, Gurero y Coleman (big thumbs up for Mike) and have them quote you a per foot price based on location. Many of the suggestions have raised valid points, distance from town, depth of water, etc. If you go with PVC/ReBar/Grout pilings be sure to specify pvc caps and any electrical you might want for lighting. You will be required to put a light at the end of your dock for navigational safety. Stainless bolts nuts and other structural hardware is a necessity not a luxury if you want any degree of longevity.