ABS/Trac control Olds Silhouette

I have a 2001 Olds Silhoutte and all of a sudden I am getting a Service traction control light and my ABS light is coming on. This started yesterday but I noticed a pattern. Today I found that when I start the van, the light is not on. As long as I make left turns the lights will not come on. As soon as I make a right turn, I hear a strange sound and then the service trac, ABS and Trac off lights come on. They stay on even though I do not hear the noise any longer. The brakes seem to work fine. I'm guessing from what I have read, this may be a sensor problem. Any rough estimates on cost?