You may not need to change the shifters, I know you don't with Campy. I went from a double to a triple, I got it used which didn't cost much. I changed the front and rear dr and of couse the crank. I sold the old stuff off so everything cost me about $250. It has worked well for me but now I am getting to the point where I don't use the small chain ring at all. If I were to buy a new bike today. I would buy a compact double, just because it weighs less.

Maybe you can buy a 12/28 cassette. IRD even makes a 12/30.