What's contaminating the coolant, and why the huge spread on coolant flush cost?

What is your question?

I've continuously been adding coolant to my car and every time I look back in it I see there is a gray/beige sludge-like substance on the cap and in the tank. What could be leaking in to cause this? Oil has been suggested to me, and if that's it, how costly is it to repair a leak like that? Another thing: Jiffy Lube charges $80 for a coolant flush, and Aamco charges $170. Is there any reason to take it to Aamco and pay more than twice what Jiffy Lube charges?