So my chain has been loose for the past week or so. Sunday, I tightened it (went as far back as there were notches) and it seemed perfect. Now, as I was riding today, I notice that it is loose again.

I checked the adjustment position and it is the same as it was when I tightened it. The axle nut is still at standard tq.

Is there anything else I am missing or is it time for a new chain?

If it is time for a new chain, how much can I expect to pay for one at say Sunrise or Cycle world?