Complete flush of the system, a new caliper if one is needed, all new pads at a quality expensive shop, I would expect to be around $500. Use the internet to see what customers say about the other shops in the area and try one of them.
ABS sensor problems need to be tracked when the light is on usually. I have had Fords with a cracked sensor that would trigger the light when water got inside and go off when they would dry out. The big problem was having it stay tripped until I got a reading to find out which one it was.