Pegasus took out 3 base stars. Expensive trade but worth it I guess.
It cost the Pegasus, but the cost would have been higher still. Without Pegasus, there is no way the colonists get off the planet.
I think Baltar was trying for redemption, telling the Cylons to take the centurions and leave before the attack was underway. I can't figure why anybody followed that obvious nutcake anyway.