If you can charge more for Fair Trade, go ahead. That depends on your customers.
In the research I've done with our customers (about 6000 of them) what matters more than fair trade is friendly customer service, time in line, and the taste of the coffee. And of course, location, location, location. Parking spaces, convenience. The basics.
There are over 350 FT certified roasters and thousands of locations that serve FT coffee.
FT reallly doesn't provide much differentiation unless you're first. And if you are doing well with it, you can expect your competition to clap on and copy you. It comes back to service, ambiance, taste and clean bathrooms. In terms of the price you charge, you're much better off thinking in terms of being the kind of place people want to be.
It has to fit with your store. Are you a place that already caters to students, etc.? Do you already connect with your community? Do you volunteer? Those coupled with FT can help improve the integrity of your position in the market.
Sorry, FT isn't a silver bullet at retail.