I don't know about your specific situation, but this might help you as a starting point. A few years ago, I switched my regular double to a compact double. It cost $250. It was all Ultegra, and I bought the parts new from the shop that put them on.
Will you be doing the work yourself? Just FYI, some shops frown on buying off the internet and having them install parts. Some shops adjust their labor charges accordingly for people who do not buy their parts from them, so it might be more cost effective to just buy the parts from the shop that will be doing the work.