Yes, just about any island is going to be more expensive since just about everything has to come in by boat. And, tourist islands have that added extra. BUT the wind in the summer in the southern Caribbean is going to be way way more consistent than Miami. Florida in the summer can be light much of the time. The best wind in FL is late fall thru spring, and it is mostly due to frontal systems moving through.
Maybe a combo of a few weeks in Bonaire and then someplace like Hatteras? I'll also suggest contacting Ann Phelan who is a windsurfer - travel agent, Caribbean Wind and Sun Vacations, www.bonaire.caribbean.com
, who knows the best and least expensive places and often can arrange some great deals. She lives in Massachusetts and Bonaire. (rough life...). Tell her I sent you and see if she can help out.
Given the wind in Florida in summer (everyone heads north), and the ideal conditions in Bonaire, I'd go to Bonaire. Even the northern Caribbean is weak for wind in summer.