First, if you posted the code(s) that were read it would help answer your questions.
Second, I don't know where you got your price, which I assume included installation, but the sensors can be purchased for considerably less. Try Canadian Tire or a NAPA store.
Third, they do not have to be replaced in pairs.
In regards your general query, faulty O2 sensors will reduce power and fuel economy and may have a detrimental effect on cayalytic.
